alum
noun
a crystalline solid, aluminum potassium sulfate, K 2 SO 4 ⋅Al 2 (SO4 ) 3 ⋅24H 2 O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.
alum
1 American
[al-uhm]
/ ˈæl əm /
noun
Chemistry.
-
Also called potassium alum, potash alum. a crystalline solid, aluminum potassium sulfate, K 2 SO 4 ⋅Al 2 (SO4 ) 3 ⋅24H 2 O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.
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one of a class of double sulfates analogous to aluminum potassium sulfate, as aluminum ammonium sulfate, having the general formula R 2 SO 4 ⋅X 2 (SO4 ) 3 ⋅24H 2 O, where R is a univalent alkali metal or ammonium, and X one of a number of trivalent metals.
-
(not in technical use) aluminum sulfate.
alum
2
American
[uh-luhm]
/ əˈlʌm /
noun
Informal.
alum.
3
American
abbreviation
Chemistry.
alum
British
/ ˈæləm /
noun
-
Also called: potash alum. a colourless soluble hydrated double sulphate of aluminium and potassium used in the manufacture of mordants and pigments, in dressing leather and sizing paper, and in medicine as a styptic and astringent. Formula: K 2 SO 4 .Al 2 (SO 4 ) 3 .24H 2 O
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any of a group of isomorphic double sulphates of a monovalent metal or group and a trivalent metal. Formula: X 2 SO 4 .Y 2 (SO 4 ) 3 .24H 2 O, where X is monovalent and Y is trivalent
alum
Scientific
/ ăl′əm /
Usage
What’s the difference between alum, alumnus, and alumni? The informal word alum refers to a graduate of a school, such as a high school or university. It’s a shortening of alumnus or alumna. In Latin, alumnus specifically refers to a male graduate, and sometimes this distinction is carried into English, with alumna being used to refer to a female graduate. The plural of alumna is alumnae. The plural of alumnus is alumni (which follows the plural ending construction used in other Latin-derived words, like stimulus and stimuli). Still, alumnus and alumni are both commonly used in a gender-neutral way. Alum is also used regardless of gender. It’s sometimes pluralized as alums. Here’s an example of alum and alumni used correctly in the same sentence. Example: As an alum, you share something with all of the alumni, regardless of when each of you graduated. Want to learn more? Read the breakdown of the difference between alumnus and alumni.
Gender
What's the difference between alum, alumnus, and alumna? See alumnus.
Etymology
Origin of alum1
First recorded in 1275–1325; Middle English < Anglo-French, from Latin alūmen; replacing Old English alefne, ælifnæ, from Old Welsh (compare Medieval Welsh elyf ) < Latin alūmini- (stem of alūmen )
Origin of alum2
First recorded in 1875–80; by shortening
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
See Examples For:
Director of Influencer Marketing Madison Love, an alum of HelloFresh and Gymshark, joined them last month.
From The Wall Street Journal ● Oct. 13, 2026
With “Digger,” Iñárritu commands — to quote “Real Housewives of New York” alum Kelly Bensimon — “You come up to me; I don’t go down to you.”
From Salon ● Oct. 5, 2026
“You feel yourself seen and reflected in the other people around you,” said actor Matthew Lillard, a Waters film alum and guest counselor.
From The Wall Street Journal ● Sep. 20, 2026
When Cameron isn’t practicing his moves with pro partner Sharna Burgess, the “Bachelorette” alum has a very personal real estate project to retreat to: his extensively renovated home in Jupiter, Fla.
From MarketWatch ● Sep. 8, 2026
Judging from his outfit, I’d say he must also be a Chumley Prep booster or alum.
From "Shine!" by J.J. and Chris Grabenstein
“I’ve always said I’m an alum. It was weird saying that.”
From Seattle Times ● May 5, 2022
“I feel it as a coach. I’m an alum. I’m a Stanford guy. And Big Game is always Big.”
From Washington Times ● Nov. 18, 2016
Mines: of alum. of Aroa. of Buria. of Caracas. of Columbia. of copper. of emeralds. deserted. of gold. of Granada. of Guanaxuato. of Los Teques. of Santa Rosa. of silver.
From Personal Narrative of Travels to the Equinoctial Regions of America, During the Year 1799-1804 — Volume 3 by Alexander von Humboldt
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
