The Black Fast is a severe form of Catholic fasting. It is the most rigorous in the history of church legislation and is marked by austerity regarding the quantity and quality of food permitted on fasting days as well as the time when such food is legitimately taken. Traditionally, the Black Fast was undertaken during Black Lent and for a prescribed period of time preceding ordination.
The details of the fast, as they were prior to the tenth century, are as follows:
- No more than one meal per day was permitted
- Flesh meat, eggs, butter, cheese and milk were forbidden
- The meal was not allowed until after sunset
- Alcohol was forbidden
- During Holy Week, the meal consisted exclusively of bread, salt, herbs, and water
Some Eastern Catholics perform the Black Fast on Fridays during Lent, and especially on Good Friday. The Black Fast is still observed by the Eastern Orthodox on Wednesdays and Fridays and during the 40 days of Lent and three other fasting periods of the year.
The term "Black Fast" has a different connotation with writers within the classical Pentecostal movement. A Black fast is complete abstinence from food or water and nothing is consumed in its duration. Dr. Curtis Ward teaches that the Black Fast should never extend beyond three days because of ketosis, possible kidney damage, and dehydration. He further states that in no New Testament scriptures did they extend this type of fast beyond that limitation and that Christ's fast included water because "he was afterward an hungred" and was offered bread. If he had abstained from water he would have obviously craved water first and foremost. Dr. Ward states that the Black Fast, Hebrew Fast, and the Absolute Fast are synonymous terms. The former Arthur Wallis coined the term "Absolute Fast" in 1968 in his book "God's Chosen Fast."  A Normal Fast or "Complete Fast" consists of eating nothing but drinking pure water. A Partial Fast (or Daniel Fast) consists of eliminating all but one type of food or eliminating just one type of food. The Black Fast is observed on rare occasions in Pentecostal circles while the Normal Fast is most usually undertaken.
The term "Black Fast" also has a different connotation within Romanian Orthodox Churches which define it somewhat similar to the definition given by those within the realms of the classical Pentecostal movement.