Cream of tartar; COT.
Cream of tartar is the common name for potassium hydrogen tartrate and is essentially an acidic salt.
In food it is widely used for stabilizing egg whites and whipped cream, preventing sugar syrups from crystallising, reducing discoloration of boiled vegetables and is also used as a component of baking powder, and sodium free salt substitutes.
Cream of tartar is also used in household products, such as brass and copper cleaners and bath / sink stain removers.