Kezayit, k'zayit, or kezayis (Hebrew: כְּזַיִת) is a Talmudic unit of volume approximately equal to the size of an average Talmudic-Era Israeli olive. The word itself literally means "like an olive." The rabbis differ on the precise definition of the unit:

• Rabbeinu Yitzchak (the Ri) defines it as one-half of a beytza (the volume of a Talmudic-Era chicken egg, literally meaning “egg”).^{[citation needed]}
• Maimonides specified that a 'grogeret' (dried fig) was one-third of a beytza, making this the maximum size for a kezayit, which is smaller. Rabbeinu Tam made the argument explicitly, though, using a slightly different calculation came out with a maximum definition of three-tenths.^{[citation needed]}
• 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.^{[citation needed]}

Its uses in halacha include:^{[citation needed]}

• 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)
• People exposed to at least a kezayit of the flesh of a dead body become ritually impure.