The Greek zymē refers to leaven or yeast — the fermenting agent that permeates dough and causes it to rise. In the ancient world, leaven was typically a piece of old fermented dough kept from a previous batch.
Leaven is one of the Bible's most versatile metaphors. In the Torah, leaven was excluded from Passover and grain offerings, symbolizing the removal of the old (Egypt, sin, corruption). Jesus warned against the 'leaven of the Pharisees and Sadducees' (Matthew 16:6) — the corrupting influence of false teaching that spreads invisibly. Paul uses the Passover metaphor directly: 'Get rid of the old yeast... For Christ, our Passover lamb, has been sacrificed' (1 Corinthians 5:7). Yet Jesus also uses leaven positively: the kingdom of heaven is like leaven that a woman hid in flour until it leavened the whole batch (Matthew 13:33) — the kingdom permeates and transforms from within. Context determines whether zymē represents corruption or transformative growth.