The Hebrew word for fasting is Soowm which means, to cover the mouth.
The Greek word for fasting is Nesteuo which means not to eat. Esteuo means to eat

There are more books on the physical aspect by food reformists than on the spiritual aspect by Christian writers. Fasting is the voluntary refusal of food for various purposes like, fighting for justice (Hunger strike), losing fats in the body (Dieting), cleansing the body system (Medical) and unto the Lord (Spiritual). Fasting for spiritual purposes has become a neglected discipline in the Church today. Fasting may provide us with a key to unlock doors where other keys have failed; a window opening up new horizons in the unseen world, a mighty spiritual weapon God has provided for the pulling down of strongholds to let the oppressed go free and to break every yoke. Fasting is not a substitute to repentance, forgiveness, prayer, faith or any other Christian discipline, but a supplement to make the Christian disciplines more effective and powerful. Our fasting should not result in producing the works of the flesh, but in producing the fruit of the Spirit.

Is 58:4-5 Behold, you fast only to quarrel and to fight and to hit with wicked fist. Fasting like yours this day will not make your voice to be heard on high. Is such the fast that I choose, a day for a man to humble himself? Is it to bow down his head like a rush, and to spread sackcloth and ashes under him? Will you call this a fast, and a day acceptable to the LORD?

Your fasting has to be unto the Lord, offered unto Him, it must be sanctified, set apart, different from the rest. First of all you must repent, forgive, surrender, make Jesus your Lord and walk in faith. Then when you undertake a fast, you must pray at the time you skip your meal, offering your meal to the Lord. This is called sanctifying the fast, a spiritual value is added to your fast, your fasting is unto the Lord and it is powerful. This is what makes it different from the rest of the fasts.
Is 58:6 "Is not this the fast that I choose: to loose the bonds of wickedness, to undo the thongs of the yoke, to let the oppressed go free, and to break every yoke?

We are all engaged in a war that is spiritual in nature, hence we need spiritual weapons in the combat. Carnal weapons like guns, swords, bombs etc. are of no benefit in a war that is spiritual in nature. Fasting, is one such weapon from the many that God has provided us, to break down strongholds. 
II Cor 10:4 The weapons of our warfare are not worldly but have divine power to destroy strongholds.

To some, fasting and starving are synonymous terms and they fear it will have harmful effects on the body. They oppose fasting almost instinctively. Why such attitudes to a practice that is so obviously scriptural? In the Old Testament, Moses, Joshua, David, Hannah, Esther, Elijah and in the New Testament, Anna and the early Church fasted and prayed. Fasting is a channel of power. It is one of the secrets of the early Church that seems to be almost lost today. 
Eph 5:29 For no man ever hates his own flesh, but nourishes and cherishes it, as Christ does the Church,

Lk 2:36-37 And there was a prophetess, Anna, the daughter of Phan'u-el, of the tribe of Asher; she was of a great age, having lived with her husband seven years from her virginity, and as a widow till she was eighty-four. She did not depart from the temple, worshiping with fasting and prayer night and day.

Acts 14:23 And when they had appointed elders for them in every Church, with prayer and fasting they committed them to the Lord in whom they believed.

Acts 13:2-3 While they were worshiping the Lord and fasting, the Holy Spirit said, "Set apart for me

Barnabas and Saul for the work to which I have called them." Then after fasting and praying they laid

their hands on them and sent them off.

PRIMARY PURPOSE : To humble oneself in repentance. Other attitudes accompanied with repentance are mourning, sack cloth and ashes. The words "To humble oneself" is used many times in the Bible and very often it means fasting.

Ps 69:10 When I humbled my soul with fasting, it became my reproach.

Is 58:5 Is such the fast that I choose, a day for a man to humble himself? Is it to bow down his head like a rush, and to spread sackcloth and ashes under him? Will you call this a fast, and a day acceptable to the LORD?

Ps 35:13 But I, when they were sick I wore sackcloth, I afflicted myself with fasting. I prayed with head bowed on my bosom,

II Chr 7:14 If my people who are called by my name humble themselves, and pray and seek my face, and turn from their wicked ways, then I will hear from heaven, and will forgive their sin and heal their land.

Ezra 8:21 Then I proclaimed a fast there, at the river Aha'va, that we might humble ourselves before our God, to seek from him a straight way for ourselves, our children, and all our goods.

Israel fasted on the day of atonement
The law of Moses commanded that Israel fast on the day of atonement, Yom Kippur (End of September to beginning of October). The whole of Israel fasted for 3500 years. It was the most sacred day in the Jewish calendar. The atonement was effective only for those people who accepted it through fasting. Anyone who did not fast on that day was to be cut off and was no longer to be a member of God's people. God did His part of atoning for their sins and the people had to do their part of fasting. The essential nature of fasting is renouncing the natural to invoke the supernatural.
Lev 16:29-31 "And it shall be a statute to you for ever that in the seventh month, on the tenth day of the month, you shall afflict yourselves, and shall do no work, either the native or the stranger who sojourns among you; for on this day shall atonement be made for you, to cleanse you; from all your sins you shall be clean before the LORD. It is a Sabbath of solemn rest to you, and you shall afflict yourselves; it is a statute for ever.

There are three important disciplines for a Christian to note, of which Jesus talks about in Matthew's gospel. The giving of alms, praying and fasting. Jesus puts fasting as an equally important discipline as alms giving and prayer. Fasting cannot be neglected just as prayer and almsgiving cannot be neglected, Jesus highlights the point that these disciplines have to be followed secretly, made known only to God and not to man.

Mt 6:1-18 "Beware of practicing your piety before men in order to be seen by them; for then you will have no reward from your Father who is in heaven. "Thus,when you give alms, sound no trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be praised by men. Truly, I say to you, they have received their reward. But when you give alms, do not let your left hand know what your right hand is doing, so that your alms may be in secret; and your Father who sees in secret will reward you. "And when you pray, you must not be like the hypocrites; for they love to stand and pray in the synagogues and at the street corners, that they may be seen by men. Truly, I say to you, they have received their reward. But when you pray, go into your room and shut the door and pray to your Father who is in secret; and your Father who sees in secret will reward you. "And in praying do not heap up empty phrases as the Gentiles do; for they think that they will be heard for their many words. Do not be like them, for your Father knows what you need before you ask him. Pray then like this: Our Father who art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done, on earth as it is in heaven. Give us this day our daily bread; And forgive us our debts, as we also have forgiven our debtors; And lead us not into temptation, But deliver us from evil. For if you forgive men their trespasses, your heavenly Father also will forgive you; but if you do not forgive men their trespasses, neither will your Father forgive your trespasses. "Andwhen you fast, do not look dismal, like the hypocrites, for they disfigure their faces that their fasting may be seen by men. Truly, I say to you, they have received their reward. But when you fast, anoint your head and wash your face, that your fasting may not be seen by men but by your Father who is in secret;and your Father who sees in secret will reward you.

Is 58:8,9,11,12 Then shall your light break forth like the dawn, and your healingshall spring up speedily; your righteousness shall go before you, the glory of the LORD shall be your rear guard. Then you shall call, and the LORD will answer; you shall cry, and he will say, Here I am. "If you take away from the midst of you the yoke, the pointing of the finger, and speaking wickedness, And the LORD will guide you continually, and satisfy your desire with good things, and make your bones strong; and you shall be like a watered garden, like a spring of water, whose waters fail not. And your ancient ruins shall be rebuilt; you shall raise up the foundations of many generations; you shall be called the repairer of the breach, the restorer of streets to dwell in.

In the above scripture God speaks about Ten blessings of fasting :
1 Light breaking forth - Illumination or understanding.
2 Healing. 
3 Righteousness.
4 Glory of the Lord.
5 Lord will answer - Answered prayer.
6 Guide you continually - Continual guidance.
7 Satisfaction.
8 Watered garden - Refreshing.
9 Waters fail not - Work that endures.
10 Rebuilt - Restoration.

The whole chapter of Isaiah 58 is on fasting please read it prayerfully.

Fast ye and pray

Prayer is powerful and when coupled with a sacrifice it becomes more powerful. Prayer coupled with fasting is very powerful to combat forces of evil.
Mk 9:17-29 And one of the multitude answered and said, "Master, I have brought unto thee my son, which hath a dumb spirit; And wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not." He answereth him, and saith, "O faithless generation, how long shall I be with you? How long shall I suffer you? Bring him unto me." And they brought him unto him: and when he saw him, straightway the spirit tare him; and he fell on the ground, and wallowed foaming. And he asked his father, "How long is it ago since this came unto him?" And he said, "Of a child. And ofttimes it hath cast him into the fire, and into the waters, to destroy him: but if thou canst do any thing, have compassion on us, and help us." Jesus said unto him, "If thou canst believe, all things are possible to him that believeth." And straightway the father of the child cried out, and said with tears, "Lord, I believe; help thou mine unbelief." When Jesus saw that the people came running together, he rebuked the foul spirit, saying unto him, "Thou dumb and deaf spirit, I charge thee, come out of him, and enter no more into him." And the spirit cried, and rent him sore, and came out of him: and he was as one dead; insomuch that many said, "He is dead." But Jesus took him by the hand, and lifted him up; and he arose. And when he was come into the house, his disciples asked him privately, Why could not we cast him out? And he said unto them, "This kind can come forth by nothing, but by prayer and fasting." (KJV)

Watch ye and pray

Sacrificing your sleep and praying is very powerful and recommended by Jesus 
Mt 26:41 "Watch and pray that you may not enter into temptation; the spirit indeed is willing, but the flesh is weak."

Lk 21:36 "But watch at all times, praying that you may have strength to escape all these things that will take place, and to stand before the Son of man."

Mk 13:33 Take heed, watch; for you do not know when the time will come.

Col 4:2 Continue steadfastly in prayer, being watchful in it with thanksgiving;

II Cor 6:4-7 But as servants of God we commend ourselves in every way: through great endurance, in afflictions, hardships, calamities, beatings, imprisonments, tumults, labors, watching, hunger; by purity, knowledge, forbearance, kindness, the Holy Spirit, genuine love, truthful speech, and the power of God; with the weapons of righteousness for the right hand and for the left;

Abstaining from sex and pray

Scripture talks about abstaining from sex and praying for married couples. 
I Cor 7:5 Do not refuse one another except perhaps by agreement for a season, that you may devote yourselves to prayer; but then come together again, lest Satan tempt you through lack of self-control.

Fasting helps us in the following ways:
To humble ourselves before God. 
To draw near to God.
To understand God's Word.
To find God's will and receive direction in your life Ezra 8:21
To receive Healing Is 58:8
To receive Deliverance Mk 9:17-29 
To receive God's intervention in some particular crisis II Chron 20:2-4 
To Intercede and pray on behalf of others.

Skipping one meal is roughly a 12 hour fast.
Skipping two meals consecutively is roughly a 24 hour fast
Skipping three meals consecutively is roughly a 36 hour fast.

God is a rewarder of them that diligently seek Him. Fasting, watching, abstaining from sex and doing any form of sacrifice is diligently seeking God.
Heb 11:6 And without faith it is impossible to please him. For whoever would draw near to God must believe that he exists and that he rewards those who seek him.

Before Jesus entered into His public ministry, He went through two critical experiences, first the Holy Spirit descended on Him at His Baptism and secondly He was led into the wilderness to fast for forty days. After His Baptism, the Bible says Jesus returned from the Jordan full of the Holy Spirit and Jesus returned from the wilderness in the power of the Spirit. Fasting brings the power of the Spirit. 
Lk 4:1 And Jesus, full of the Holy Spirit, returned from the Jordan, and was led by the Spirit
Lk 4:14 And Jesus returned in the power of the Spirit into Galilee, and a report concerning him went out through all the surrounding country.

TYPES OF FAST

Absolute fast
Abstaining from eating as well as drinking. Normally this was never done for more than 3 days, because any longer period might prove injurious to the physical body. Desperate situations require desperate measures. Absolute fasts are of supernatural character and God inspired. Such a fast is called an Esther fast. Such fasts must be an inspiration from God and not a temptation from the enemy. You cannot just imitate someone in such a fast, for it could lead to serious physical problems.

Ezra 10:6 Then Ezra withdrew from before the house of God, and went to the chamber of Jehoha'nan the son of Eli'ashib, where he spent the night, neither eating bread nor drinking water; for he was mourning over the faithlessness of the exiles.

Queen Esther instructed Mordecai to neither eat nor drink for three days and nights.
Esth 4:16 "Go, gather all the Jews to be found in Susa, and hold a fast on my behalf, and neither eat nor drink for three days, night or day. I and my maids will also fast as you do. Then I will go to the king, though it is against the law; and if I perish, I perish."

When Saul was on his way to Damascus to persecute Christians, he encountered the Lord and he was struck blind and it is said that when he reached Damascus he neither ate nor drank for three days. 
Acts 9:9 And for three days he was without sight, and neither ate nor drank.

Partial fast 
It is eating partially, avoiding delicacies not eating what you like. It is beneficial for all especially for those who do not keep good health and who cannot skip an entire meal. people suffering from diabetes, tuberculosis and other disease. Such a fast is called a Daniel fast.
Dan 10:2-3 In those days I, Daniel, was mourning for three weeks. I ate no delicacies, no meat or wine entered my mouth, nor did I anoint myself at all, for the full three weeks.

Normal fast
Abstaining from all food, solid or liquid but not from water for 40 days. You may wonder why such a fast is called a normal fast. Medical doctors say that a normal healthy man or woman can comfortably go without food for forty days and forty nights and during this time of fast the body lives on the fat stored. Normally in a healthy human being it takes forty days to use up the fat. If any one extends this fast beyond forty days it is dangerous, for the human body will now live on the living tissues and degeneration of tissues will take place and this is starvation and not fasting. During a fast the body eliminates toxins, that is why a lot of people fast regularly on week ends for medical reasons. 
When a person fasts for forty days, all the toxins in his body are eliminated in such a manner, that his breath smells like the breath of a baby, his breath smells sweet. Such fasts are of supernatural character and are to be undertaken only under the inspiration of the Holy Spirit confirmed by your spiritual director. During such fasts much time has to be spent in Prayer, Word of God and Rest. One has to drink plenty of fresh water, water does not break your fast. Water with a little lime juice and a dash of honey has a tremendous cleansing effect on the body. 
Prior to such fasts one has to see that the bowels are completely evacuated and clean, this can be achieved by taking an enema and must not be neglected, if you neglect evacuation of the bowels prior to a fast, the contents of the bowels will not be evacuated naturally, since there is no intake of food, hence the contents of the bowels will decay within throughout the entire course of the fast causing poisoning, disease and death. 
During a fast of this nature with no intake of food for long periods of time the stomach shrinks in size, So breaking a long fast has to be done gradually by taking fruit juices or liquid foods for a few days followed by semi solid foods for another few days and then resuming to normal foods. Be careful to see that the stomach comes to normal size gradually and not suddenly. Breaking a fast of this nature suddenly by stuffing food would be dangerous and therefore to be avoided.

Mt 4:2 And he fasted forty days and forty nights, and afterward he was hungry.

Lk 4:2 For forty days in the wilderness, tempted by the devil. And he ate nothing in those days; and when they were ended, he was hungry.
Satan tempted Him to eat, not to drink. Infact the human body cannot survive 40 days without water. 

Examples of normal fasts in the Bible.
Moses - Deut 9:9,18, Ex 34:28 - no food and water
Joshua - Ex 24:13-18, 32:15-17
Elijah - I Kings 19:8
Jesus - Mt 4:2, Lk 4:2

Sabbath and Fasting 
To seek the Lord on the Sabbath day. Fasting on the Sabbath day is scriptural.

Is 58:13-14 "If you turn back your foot from the Sabbath, from doing your pleasure on my holy day, and call the Sabbath a delight and the holy day of the LORD honorable; if you honor it, not going your own ways, or seeking your own pleasure, or talking idly; then you shall take delight in the LORD, and I will make you ride upon the heights of the earth; I will feed you with the heritage of Jacob your father, for the mouth of the LORD has spoken."

Solemn Assembly
God called the Israelites to gather together on certain occasions. A day when everyone was to stop all their own activities and take time to seek God collectively.

Joel 1:14 Sanctify a fast, call a solemn assembly. Gather the elders and all the inhabitants of the land to the house of the LORD your God; and cry to the LORD.

Joel 2:15-16 Blow the trumpet in Zion; sanctify a fast; call a solemn assembly; gather the people. Sanctify the congregation; assemble the elders; gather the children, even nursing infants. Let the bridegroom leave his room, and the bride her chamber.

II Chr 20:3-4 Then Jehoshaphat feared, and set himself to seek the LORD, and proclaimed a fast throughout all Judah. And Judah assembled to seek help from the LORD; from all the cities of Judah they came to seek the LORD.

The responsibility to humble ourselves is upon us, we cannot transfer that responsibility to God.

Humble like a child
Mt 18:4 Whoever humbles himself like this child, he is the greatest in the kingdom of heaven.

Humbles himself will be exalted
Mt 23:12 Whoever exalts himself will be humbled, and whoever humbles himself will be exalted.

Humble yourselves before the Lord and He will exalt you.
James 4:10 Humble yourselves before the Lord and he will exalt you.

I Pt 5:6 Humble yourselves therefore under the mighty hand of God, that in due time he may exalt you.

Calling for all Leaders 
There is a desperate need for men of God in leadership to lead God's people back to His pattern of collective prayer and fasting, to seek God's intervention on behalf of His people.

Joel 1:13 -14 Gird on sackcloth and lament, O priests, wail, O ministers of the altar. Go in, pass the night in sackcloth, O ministers of my God! Because cereal offering and drink offering are withheld from the house of your God. Sanctify a fast, call a solemn assembly. Gather the elders and all the inhabitants of the land to the house of the LORD your God; and cry to the LORD.

Joel 2:15 -17 Blow the trumpet in Zion; sanctify a fast; call a solemn assembly; gather the people. Sanctify the congregation; assemble the elders; gather the children, even nursing infants. Let the bridegroom leave his room, and the bride her chamber. Between the vestibule and the altar let the priests, the ministers of the LORD, weep and say, "Spare thy people, O LORD, and make not thy heritage a reproach, a byword among the nations. Why should they say among the peoples, 'Where is their God?"

By: Trevor Lewis
Jeevan Jal Ministries