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
aluminum sulfate: an erroneous use
ME & OFr < L alumen, alum: for IE base see ale
Informal an alumnus or alumna
See alum in American Heritage Dictionary 4
Any of various double sulfates of a trivalent metal such as aluminum, chromium, or iron and a univalent metal such as potassium or sodium, especially hydrous aluminum potassium sulfate, AlK(SO4)2·12H2O, widely used in industry as clarifiers, hardeners, and purifiers and medicinally as topical astringents and styptics.