an acid, HOOC(CHOH)COOH, a clear, colorless crystal or a white, crystalline powder, found in vegetable tissues and fruit juices and obtained commercially from tartar: it is used in dyeing, photography, medicine, etc.
See tartaric acid in American Heritage Dictionary 4
noun
Any of four isomeric crystalline organic compounds, C4H6O6, used to make cream of tartar and baking powder, as a sequestrant, and in effervescent beverages and photographic chemicals.