Leaven (חָמֵץ, chamets; שְׂאֹר, se'or; ζύμη, zymē). A fermented substance that causes dough to rise.

The leaven used in ancient Palestine was just a piece of fermented dough kept from a previous baking. The lump of leftover dough was either dissolved in water in the kneading-trough before the flour was added, or it was put into the flour and kneaded along with it, as described in one of Jesus’ parables (Matt 13:33). Bread made in this way was distinguished from unleavened bread, which did not rise. Leaven was supposed to be removed from Israelite houses during the Feast of Unleavened Bread (e.g., Exod 12:15, 19; 13:7). It was also forbidden in grain offerings (e.g., Exod 23:18; Lev 2:11).

The figurative uses of leaven in the New Testament sometimes imply that leaven was viewed as a corrupting substance (Matt 16:6). Paul twice says that “a little leaven leavens the whole batch of dough” (1 Cor 5:6; Gal 5:9). However, Jesus uses it once in a positive way, referring to the change caused by the kingdom of heaven (Matt 13:33).