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.
- An agent, such as yeast, that causes batter or dough to rise, especially by fermentation.
- An element, influence, or agent that works subtly to lighten, enliven, or modify a whole.
- To add a rising agent to.
- To cause to rise, especially by fermentation.
- To pervade with a lightening, enlivening, or modifying influence.