late 14c., "whitish mineral salt used as an astringent, dye, etc.," from Old French alum, from Latin alumen "alum," literally "bitter salt," cognate with Greek aludoimos "bitter" and perhaps with English ale.
alum al·um (āl'əm)
Any of various double sulfates of a trivalent metal such as aluminum or iron and a univalent metal such as potassium or sodium that are used as topical astringents and styptics.
Any of various crystalline double salts of a trivalent metal (such as aluminum, chromium, or iron) and a monovalent metal (such as potassium or sodium), especially aluminum potassium sulfate. Alum is widely used in industry as a hardener and purifier, and in medicine as an emetic and to stop bleeding.
An alumnus or alumna