Fasting is the voluntarily act of abstaining from eating food and/or liquid for a period of time.
The Bible records many examples of people who fasted. Some abstained from eating all foods, and others abstained from eating only certain kinds of foods for the duration of their fast. An example of the latter would be Daniel’s three-week fast, when he ate no “tasty food…meat or wine” (Dan. 10:3).
There are also a few examples in Scripture of people who fasted both food and water, but this kind of total fast was rare and should be considered supernatural if it lasted longer than three days. When Moses, for example, went for forty days without eating or drinking anything, he was in the presence of God Himself, to the extent that his face shined (see Ex. 34:28-29). He repeated a second 40-day fast shortly after his first one (see Deut. 9:9, 18). His were two very supernatural fasts, and no one should attempt to imitate Moses in this regard. It is impossible, apart from the supernatural help of God, for a person to survive more than a few days without water. Dehydration leads to death. Most healthy people can survive with food, however, for quite a few weeks.