'Baking powder' definitions:

Definition of 'baking powder'

noun

Any of various powdered mixtures used in baking as a substitute for yeast

From: GCIDE

Powder \Pow"der\, n. [OE. poudre, pouldre, F. poudre, OF. also poldre, puldre, L. pulvis, pulveris: cf. pollen fine flour, mill dust, E. pollen. Cf. Polverine, Pulverize.]
1. The fine particles to which any dry substance is reduced by pounding, grinding, or triturating, or into which it falls by decay; dust. [1913 Webster]
Grind their bones to powder small. --Shak. [1913 Webster]
2. An explosive mixture used in gunnery, blasting, etc.; gunpowder. See Gunpowder. [1913 Webster]
Atlas powder, Baking powder, etc. See under Atlas, Baking, etc.
Powder down (Zool.), the peculiar dust, or exfoliation, of powder-down feathers.
Powder-down feather (Zool.), one of a peculiar kind of modified feathers which sometimes form patches on certain parts of some birds. They have a greasy texture and a scaly exfoliation.
Powder-down patch (Zool.), a tuft or patch of powder-down feathers.
Powder hose, a tube of strong linen, about an inch in diameter, filled with powder and used in firing mines. --Farrow.
Powder hoy (Naut.), a vessel specially fitted to carry powder for the supply of war ships. They are usually painted red and carry a red flag.
Powder magazine, or Powder room. See Magazine, 2.
Powder mine, a mine exploded by gunpowder. See Mine.
Powder monkey (Naut.), a boy formerly employed on war vessels to carry powder; a powder boy.
Powder post. See Dry rot, under Dry.
Powder puff. See Puff, n. [1913 Webster]

From: GCIDE

Baking \Bak"ing\, n.
1. The act or process of cooking in an oven, or of drying and hardening by heat or cold. [1913 Webster]
2. The quantity baked at once; a batch; as, a baking of bread. [1913 Webster]
Baking powder, a substitute for yeast, usually consisting of an acid, a carbonate, and a little farinaceous matter. [1913 Webster]