Definition of "alum" []

• 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 aluminum potassium sulfate, AlK(SO4)2·12H2O, widely used in industry as clarifiers, hardeners, and purifiers and medicinally as topical astringents and styptics. (noun)
• Informal An alumna or alumnus. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The Lost alum is set to guest-star on How I Met Your Mother, a show rep tells TVGuide. com."
• "The former Merck executive and Fordham University alum is giving $1 million to build a new football locker room on the university's Rose Hill campus in the Bronx."
• "After a shortlived gig on the canned CW drama The Beautiful Life, the High School Musical alum is sliding back into his singing and dancing shoes to leap to “new heights” in the Tony-award winning musical."