1. To add a leavening agent (such as yeast) to; to cause to ferment.
2. (Archaic) To imbue; to infect; to vitiate.

ETYM Old Eng. levain, levein, French levain, Latin levamen alleviation, mitigation; but taken in the sense of, a raising, that which raises, from levare to raise. Related to Lever.
1. A substance used to produce fermentation in dough or a liquid; SYN. leavening.
2. An influence that works subtly to lighten or modify something; SYN. leavening.
Element inducing fermentation. The term is applied to the yeast added to dough in bread making; it is used figuratively to describe any pervasive influence, usually in a good sense, although in the Old Testament it symbolized corruption, and unleavened bread was used in sacrifice.
During the Jewish festival of Pesach (Passover), all leaven is removed from the house.