Leaven

Webster's Practical Dictionary- A Practical Dictionary of the Englih Language · 1884 · p. 235

lev'n, n. A portion of fermenting dough, which, mixed with a larger quantity, produces fermentation in it, and renders it light; yeast; barm; anything which produces fermentation or works general change in the ma .— v. t. [leavened (-nd), -ening.] To excite fermentation in; to taint, imbue.