baking powder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A mixture of baking soda, starch, and at least one slightly acidic compound such as cream of tartar that works as a leavening agent in baking by releasing carbon dioxide when mixed with a liquid, such as milk or water.

from The Century Dictionary.

• noun Any powder used as a substitute for yeast in raising bread, cakes, etc.
• noun The baking-powders in use in the United States belong virtually to the following classes: bicarbonate of soda with cream of tartar
• noun acid calcium phosphate
• noun dry soda-alum
• noun both acid calcium phosphate and dry soda-alum; and in all starch to absorb hygroscopic moisture from the air and prevent premature loss of carbon-dioxid gas.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a dry leavening agent used in baking; a mixture of baking soda, an acid (often cream of tartar) and starch to keep it dry; when wetted, carbon dioxide is released

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various powdered mixtures used in baking as a substitute for yeast