A double sulfate of aluminum and of an alkaline earth element or ammonium; chemically, an alum is any one of the markedly astringent double salts formed by a combination of a sulfate of aluminum, iron, manganese, chromium, or gallium with a sulfate of lithium, sodium, potassium, ammonium, cesium, or rubidium; used locally as a styptic. [L. alumen]