al·um 1 
(ăləm)
(ăləm)n.
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.
[Middle English, from Old French, from Latin alūmen.]