Leaven

lev′n, n. the ferment which makes dough rise in a spongy form: anything that makes a general change, whether good or bad.— v.t. to raise with leaven: to taint.— n. Leav′ening .— adj. Leav′enous , containing leaven. [Fr. leva in —L. levamen — levāre , to raise— levis , light.]