Ke'zayit

Ke'zayit ( _he. כזית) is a Talmudic unit of volume approximately equal to the size of an average olive. The word itself literally means "like an olive." The rabbis differ on the precise definition of the unit:
* Rashi defines it as one-half of a beytza (a beytza is the volume of an egg).
* Rambam defines it as one-third of a beytza.
* According to some interpretations, including the Chazon Ish, the "zayit" is not related to other units by a fixed ratio, but rather should only be conceived of independently as the size of an average olive.

Its uses in halacha include:

* The minimum amount food that, when eaten, is halachically considered "eating." This has implications throughout the spectrum of halacha, including:
** For prohibitions of consumption, as in the eating of "milk and meat"
** For the saying of a Bracha Ahrona (the traditional grace after meals)
* If a person is exposed to at least a "ke'zayit" of the flesh of a dead body, he or she becomes ritually impure.

ee also

* Ancient Hebrew units of measurement
* Olive (fruit)