Leaven— (1.) Heb. seor (Ex. 12:15, 19; 13:7; Lev. 2:11), the remnant of dough from the preceding baking which had fermented and become acid.
(2.) Heb. hamets, properly "ferment." In Num. 6:3, "vinegar of wine" is more correctly "fermented wine." In Ex. 13:7, the proper rendering would be, "Unfermented things [Heb. matstsoth] shall be consumed during the seven days; and there shall not be seen with thee fermented things [hamets], and there shall not be seen with thee leavened mass [seor] in all thy borders." The chemical definition of ferment or yeast is "a substance in a state of putrefaction, the atoms of which are in a continual motion."
The use of leaven was strictly forbidden in all offerings made to the Lord by fire (Lev. 2:11;7:12;8:2; Num. 6:15). Its secretly penetrating and diffusive power is referred to in 1 Cor. 5:6. In this respect it is used to illustrate the growth of the kingdom of heaven both in the individual heart and in the world (Matt. 13:33). It is a figure also of corruptness and of perverseness of heart and life (Matt. 16:6, 11; Mark 8:15; 1 Cor. 5:7, 8).