Fasting is primarily a movement of willing abstinence or reduction from certain or all food, drink, or both, for a period of time.An absolute fast 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.Other fasts may be only partially restrictive, limiting particular foods or substances. The fast may also be intermittent in nature.n absolute fast 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. Other fasts may be only partially restrictive, limiting particular foods or substances. The fast may also be intermittent in nature.