A substance, such as yeast, used to produce fermentation in dough to cause it to rise or lighten.

Webster’s Dictionary:

n.

1. An agent, such as yeast, that causes batter or dough to rise, especially by fermentation.

2. An element, influence, or agent that works subtly to lighten, enliven, or modify a whole.

tr.v. leav•ened, leav•en•ing, leav•ens

1. To add a rising agent to.

2. To cause to rise, especially by fermentation.

3. To pervade with a lightening, enlivening, or modifying influence.