Fasting is simply voluntary abstinence from food for spiritual purposes. It is the opportunity to express to God in a very tangible way that we need Him more than anything else, even a physical meal. Fasting is always accompanied by focused prayer.
How Long is a Fast?
When you read the Bible, you will find various time-lengths of fasting. Esther, a Jewish Queen, called the people of Israel to fast for three days when she was facing a challenging situation (Esther 4:16). Daniel fasted for three weeks after receiving a vision from God (Daniel 10:2-3). The prophet Joel called the nation to one day of fasting and prayer in response to God’s judgment of their sin (Joel 1:14; 2:15). Jesus fasted for forty days as a way of preparing for His public ministry (Matthew 4:1-3). The Apostle Paul fasted for three days after encountering the risen Jesus on the road to Damascus (Acts 9:9). The church at Antioch launched Paul and Barnabas out on their church-planting mission after a short time of prayer and fasting (Acts 13:2).
What Food or Drink is Involved in Fasting?
While there are different types of fasts in Scripture, fasting usually involves abstinence from all food and drink except water. This is typically called a ‘total fast’. There are some places in the Bible where people participate in partial fasts for a variety of reasons. The most memorable is Daniel and his colleagues who abstained from the royal foods in Babylon for spiritual reasons (Daniel 1:8-14).
If you are new to fasting, Richard Foster (author of Celebration of Discipline) offers some very helpful advice about: start easier with a partial fast (some food and juices to drink) and then work your way up to a total fast from food with only water to drink.
Quotations on Fasting
Here are a series of definitions of fasting by respected leaders in the church:
“Abstaining from food for spiritual purposes.” – Richard Foster
“Fasting is the natural, inevitable response of a person to a grievous sacred moment in life.” – Scot McKnight
“A fast is the self-denial of normal necessities in order to intentionally attend to God in prayer. Bringing attachments and cravings to the surface opens a place for prayer. This physical awareness of emptiness is the reminder to turn to Jesus who alone can satisfy.” – Adele Ahlberg Calhoun
“Simply defined fasting is the act of doing without something in order to put more focus on God.” – David Drury.
“Fasting, if we conceive of it truly, must not only be confined to the question of food and drink; fasting should really be made to include abstinence from anything which is legitimate in and of itself for the sake of some special spiritual purpose. There are many bodily functions which are right and perfectly legitimate, but which for special peculiar reasons in certain circumstances should be controlled. That is fasting. There, I suggest, is a kind of general definition of what is meant by fasting.” – Martyn Lloyd-Jones.
“The self-denial of normal necessities in order to intentionally attend to God in prayer. Usually associated with food, but it could be from people, the media, phone, sleep, or even shopping.” – Steve Sonderman (in sermon “Becoming a Person of Prayer”)
 David Drury, The Fruitful Life: What Will I Be Remembered For?, 2nd ed. (Spring Lake, MI: Spring Lake Wesleyan Church, 2004), 128.