cream of tartar, white crystalline powder. Chemically it is potassium hydrogen tartrate, KC₄H₅O₆, the acidic potassium salt of tartaric acid. It is used as the leavening agent in baking powders. An impure form, called tartar or argol, forms naturally during the fermentation of grape juice into wine and crystallizes in the wine casks.

Uses

Also known as tartaric acid, real cream of tartar is derived only from the crust on wine casks, where it has precipitated from the tartaric acid in the grapes. It has an astringent quality which adds a tartness to foods. To mix your own baking powder, combine 1/4 tsp. baking soda with 1/2 tsp. cream of tartar. When added to meringues for pies or other baked egg white products, cream of tartar helps maintain the fluffiness. Also a good agent with which to clean copper cookware.

Re: what is cream of tarter

Cream of tartar is a acid compound, chiefly potassium bitartrate, found in the juice of grapes and deposited on the sides of casks during winemaking

It's often added to frosting mixtures for a creamier consistency, and to egg whites before beating to improve stability and volume. It's used as the acid ingredient in many baking powders.