A ta'anit or taynis (Biblical Hebrew תַּעֲנִית taʿaniṯ or צוֹם ṣom) is a fast in Judaism in which one abstains from all food and drink, including water.

Purposes

A Jewish fast may have one or more purposes, including:

Jewish fast days

Full fast

A Jewish full fast lasts from sunset to darkness the following night. There are two Jewish full fast days: The two full fast days carry four restrictions in addition to eating and drinking – one may not wash one's body, wear leather shoes, use colognes, oils or perfumes, or have sexual relations. Yom Kippur also has all the restrictions of Shabbat, and Tisha B'Av has restrictions somewhat similar to a mourner sitting shiva. The Halakha status of the two Jewish full fasts is that they are obligatory.

Minor fasts

Minor fasts are observed from dawn to nightfall, without additional restrictions. There are four public minor fasts: There are additional fasts that are practiced in some communities or by individuals, but are not universally observed like the ones listed above. Since these are dependent on local custom, it is impossible to give a comprehensive list. Nevertheless, some of the most commonly observed ones include:

Liturgy

During the four minor fasts a number of changes is made to the liturgy:

Four fasts

The major and minor fasts that commemorate events having to do with the destruction of the Jerusalem Temple are called the four fasts. They are: The minor fasts are mentioned in the Bible as fasts in memory of the destruction of the First Temple. However, after the Second Temple was built, these fasts ceased to be observed. The Talmud establishes general rules for observance of the fasts in later periods: if the Temple stands the fasts are not observed and instead have the status of Yom Tov and observed as holidays; if the Jewish people are being persecuted the fasts are observed; if neither of those is the case, then "should they desire, they fast, should they desire not to, they do not fast." Nowadays, the Jewish people are accustomed to observing these fasts, making them obligatory.

Customary fasts

Customary fasts are only practiced by specific communities, or by especially pious individuals, or by certain classes of individuals. Most of these fasts, like the minor fasts, last from dawn to dusk. Customarily, special prayers called selichot are added in the morning prayer services on many of these days.

Breaking the fast

A break fast is a meal that takes places following a fast. After Yom Kippur, it is viewed as a festive meal. To avoid indigestion, some choose to avoid heavy foods such as meat, observe a custom of eating light dairy foods in moderation.

Other abstentions from food

From the Gemara there is a prohibition against eating before Shacharit, the morning prayers, except for those who are ill or unable to concentrate.