Easton's Bible Dictionary: M. G. Easton, M.A., D.D.

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 (Mt. 13:33). It is a figure also of corruptness and of perverseness of heart and life (Mt. 16:6, 11; Mk. 8:15; 1 Cor. 5:7, 8).