baking powder

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Century Dictionary and Cyclopedia

1. n. Any powder used as a substitute for yeast in raising bread, cakes, etc. Baking-powders are composed of bicarbonate of sodium or potassium mixed with a dry powder capable of setting carbonic acid free when the mixture is moistened.
2. n. The baking-powders in use in the United States belong virtually to the following classes: bicarbonate of soda with cream of tartar
3. n. acid calcium phosphate
4. n. dry soda-alum
5. n. 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.

Wiktionary

1. n. 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

GNU Webster's 1913

1. n. a substitute for yeast, usually consisting of an acid, a carbonate, and a little farinaceous matter.

WordNet 3.0

1. n. any of various powdered mixtures used in baking as a substitute for yeast