al|um

al|um¹ «AL uhm», noun.

1. a whitish mineral salt used in medicine and in dyeing; potash alum. A pencil of alum is sometimes used to stop the bleeding of a small cut. Formula: KAI(SO₄)₂•12H₂O

2. a colorless, crystalline salt containing ammonia, used in baking powder, foam fire extinguishers, the purification of drinking water, and in medicine; ammonia alum. Formula: NH₄Al(SO₄)₂•12H₂O

3. any one of a group of double salts analogous to and including potash alum, formed by the union of the sulfate of a trivalent metal, such as aluminum, chromium, or iron, with the sulfate of a monovalent alkali metal or ammonium.

4. U.S. = aluminum sulfate. (Cf. ↑aluminum sulfate)

╂[< Old French alum < Latin alūmen]

al|um² «AL uhm», noun.

a graduate or former student of a particular school: »

The theater was filled with Shakespeare Academy alums.

╂[ shortening of alumnus]