the.com/leavening

means An agent like yeast or baking powder that makes dough or batter rise by releasing gas through it, or, more broadly, anything added in small amount to lighten, lift, or enliven a heavier whole.

from From Old French levain, rooted in Latin levare, 'to raise or lighten' — the same ancestor that gives English 'lever' and 'elevate.' The deep root is Latin levis, 'light in weight,' so the word has always carried its meaning in its bones: whatever leavens is, etymologically, the thing that makes something less heavy. The metaphorical sense — a little influence that lifts or transforms a larger mass, as in 'leavening the discussion with humor' — rides straight off the baking image and is old enough to appear in biblical translations, where a small amount of leaven worked through the whole lump.