Anything that in cooking produces fermentation, a lump of old dough being generally used. No leaven was allowed during the Passover Feast (Ex. 12:15, 19; 13:7; Deut. 16:4) or in offerings made to God with fire (Ex. 29:2; Lev. 2:11; 7:12; Num. 6:15). It was probably forbidden because there was associated with it the idea of corruption. In the New Testament it was as a rule symbolic of sin (Matt. 16:6; Mark 8:15; Luke 12:1; 1 Cor. 5:7; Gal. 5:9); but in Matt. 13:33 it is compared to the inward growth and influence of the kingdom of heaven (church) upon the earth.