From Wikipedia, the free encyclopedia. Fasting is a willing abstinence or reduction from some or all food, drink, or both, for a period of time. An absolute fast (dry fasting) is normally defined as abstinence from all food and liquid for a defined period, usually a period of 24 hours, or a number of days. Water fasting allows drinking water but nothing else. Other fasts may be partially restrictive, limiting only particular foods or substances.