(1.) Hebrews seor (Exodus 12:15, 19; 13:7; Leviticus 2:11), the remnant of dough from the preceding baking which had fermented and become acid.

(2.) Hebrews hamets, properly "ferment." In Numbers 6:3, "vinegar of wine" is more correctly "fermented wine." In Exodus 13:7, the proper rendering would be, "Unfermented things [Hebrews 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 (Leviticus 2:11; 7:12; 8:2; Numbers 6:15). Its secretly penetrating and diffusive power is referred to in 1 Corinthians 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 (Matthew 13:33). It is a figure also of corruptness and of perverseness of heart and life (Matthew 16:6, 11; Mark 8:15; 1 Corinthians 5:7, 8).

Noah Webster's New International Dictionary of the English Language

1. (n.) Yeast or any substance that causes dough to rise, usually by fermentation.

2. (n.) Anything which makes a general assimilating (especially a corrupting) change in the mass.

3. (v. t.) To make light by the action of leaven; to cause to ferment.

4. (v. t.) To imbue; to infect; to vitiate.