Kezayit

• Rabbeinu Yitzchak defines it as one-half of a beytza.
• Rambam specified that a 'grogeret' 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.
• 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.

• 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
• If a person is exposed to at least a kezayit of the flesh of a dead body, he or she becomes ritually impure.