Real Dictionary

baking powder
variable noun: Baking powder is an ingredient used in cake making. It causes cakes to rise when they are in the oven.

baking powder in American English
a leavening agent that raises dough by the gas (carbon dioxide) produced when baking soda and acid react in the presence of water: it usually contains baking soda mixed with either starch or flour and cream of tartar or another acid-forming substance, as
noun: any of various powders used as a substitute for yeast in baking, composed of sodium bicarbonate mixed with an acid substance, as cream of tartar, capable of setting carbon dioxide free when the mixture is moistened, causing the dough to rise

baking powder in British English
noun: any of various powdered mixtures that contain sodium bicarbonate, starch (usually flour), and one or more slightly acidic compounds, such as cream of tartar: used in baking as a substitute for yeast