Leav"en (?), n. [OE. levain, levein, F. levain, L. levamen alleviation, mitigation; but taken in the sense of, a raising, that which raises, fr. levare to raise. See Lever, n.] 1. Any substance that produces, or is designed to produce, fermentation, as in dough or liquids; esp., a portion of fermenting dough, which, mixed with a larger quantity of dough, produces a general change in the mass, and renders it light; yeast; barm.

2. Anything which makes a general assimilating (especially a corrupting) change in the mass.

Beware of the leaven of the Pharisees, which is hypocrisy.

Leav"en, v. t. [imp. & p. p. Leavened (?); p. pr. & vb. n. Leavening (?).] 1. To make light by the action of leaven; to cause to ferment.

A little leaven leaveneth the whole lump.

2. To imbue; to infect; to vitiate.

With these and the like deceivable doctrines, he leavens also his prayer.

• any agent used to make dough rise or to have a similar effect on baked goods
  
• to add a leavening agent
  
• to cause to rise by fermentation