alum

noun

1. a double sulfate of a monovalent metal or radical (as sodium, potassium, or ammonium) with a trivalent metal (as aluminum, iron, or chromium): it is used as an astringent, as an emetic, and in the manufacture of baking powders, dyes, and paper: the commonest form is potash alum (potassium aluminum sulfate), KAl(SO)·12HO
2. aluminum sulfate: an erroneous use

noun