Cream of tartar

[Julene]

An acidic salt-potassium hydrogen tartrate (also referred to as tartaric acid); stabilizes beaten egg whites and leavens some baked goods.

[Sommer]

a white powder used in baking

[Dave Paprocki]

a salt used especially in baking powder

[Urukdaf 0]

Potassium bitartrate, also known as potassium hydrogen tartrate, with formula KC4H5O6, is a byproduct of winemaking. In cooking, it is known as cream of tartar. It is processed from the potassium acid salt of tartaric acid (a carboxylic acid). The resulting powdery base can be used in baking or as a cleaning solution (when mixed with an acidic solution such as lemon juice or white vinegar).