Definition of: leaven

(lev′ən) noun
1. Fermenting dough, or anything that causes fermentation, as yeast.
2. Any influence or addition that causes general change or modification of the whole.
—v.t.
1. To produce fermentation in; make light by fermentation.
2. To affect in character; imbue. [<OF levain <L levamen alleviation <levare raise]
—leav′en·ing noun