Leaven. Any substance that promotes fermentation. Sour dough is generally used in the East for this purpose; lees of wine are also employed. The fermentation produced is a kind of putrefaction; indeed it is distributed into three kinds, the vinous, the acetous, the putrefactive. All leaven was prohibited in meat-offerings, Leviticus 2:11; Leviticus 7:12; Leviticus 8:2; Numbers 6:15, and specially in the paschal feast of the Hebrews, Exodus 12:3; Exodus 12:19-20; whence this was often called "the feast of unleavened bread." Matthew 26:17. The nature of leaven, affecting the whole lump of the substance to which it is added, furnishes some striking illustrations in Scripture, Matthew 13:33; Matthew 16:6; 1 Corinthians 6:6; as also does the corruption it had undergone; thus we have warnings in Luke 12:1; 1 Corinthians 5:7-8, where the word is symbolically used for corruptness of life, or doctrine.