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; Ex. 13: 7; Deut. 16: 4), or in offerings made to God with fire (Ex. 29: 2; Lev. 2: 11; Lev. 7: 12; Num. 6: 15). It was probably forbidden because there was associated with it the idea of corruption. In the N.T. it was a rule symbolical 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.