The meaning of Leaven in the Bible
(From Easton's Bible Dictionary)

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

Heb. 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 [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 (Leviticus 2:11; 7:12; 8:2; Numbers 6:15). Its secretly penetrating and diffusive power is referred to in I 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; I Corinthians 5:7,8).

See more on the meaning of Leaven in the Bible:
Leaven {International Standard Bible Encyclopedia}

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