leaven

Concise Oxford English Dictionary © 2008 Oxford University Press:
leaven /ˈlɛv(ə)n/

▶noun

• 1 a substance, typically yeast, added to dough to make it ferment and rise.

  ■ dough reserved from an earlier batch in order to start a later one fermenting.

• 2 a pervasive influence that modifies something or transforms it for the better.

▶verb

• 1 (usu. as adj. leavened) cause (dough or bread) to ferment and rise by adding leaven.

• 2 permeate and modify or transform for the better.

– origin ME: from OFr. levain, based on L. levamen ‘relief’ (lit. ‘means of raising’), from levare ‘to lift’.