Yeast, added to dough. In the Bible leaven had two incompatible significances: as fermented dough left over unbaked from a previous batch it was added to cause further fermentation. It could therefore be regarded either as a living substance which should not be burnt in sacrifices on the altar or as a corrupting influence which caused a kind of decay. The latter was the understanding of the rabbis: leaven symbolized the corruption of human nature. Hence Jesus' condemnation of the leaven (‘yeast’, NRSV) of Pharisees and others (Mark 8: 15) and Paul's denunciation in 1 Cor. 5: 6–8 of the Corinthians' arrogance and in Gal. 5: 9 of the wiles of his opponents. On the other hand leaven as life‐giving is used in the parables of Matt. 13: 33 and Luke 13: 20–21.

During the feast of Unleavened Bread, which became combined with the Passover, Jews ate only unleavened bread (Exod. 13: 7–8).