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alum

1
[ al-uhm ]
/ ˈæl əm /
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noun Chemistry.
Also called potash alum, potassium alum. a crystalline solid, aluminum potassium sulfate, K2SO4⋅Al2(SO4)3⋅24H2O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.
one of a class of double sulfates analogous to aluminum potassium sulfate, as aluminum ammonium sulfate, having the general formula R2SO4⋅X2(SO4)3⋅24H2O, where R is a univalent alkali metal or ammonium, and X one of a number of trivalent metals.
(not in technical use) aluminum sulfate.
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Origin of alum

1
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 )

Other definitions for alum (2 of 3)

alum2
[ uh-luhm ]
/ əˈlʌm /

noun Informal.
an alumna or alumnus: Nearly 45 of us alums went on a day trip on the river to celebrate graduation.

Origin of alum

2
First recorded in 1875–80; by shortening

usage note for alum

See alumnus.

WORDS THAT MAY BE CONFUSED WITH alum

alum , alumna, alumnae, alumni, alumnus

Other definitions for alum (3 of 3)

alum.

abbreviation Chemistry.
aluminum.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022

ALUM VS. ALUMNUS VS. ALUMNI VS. ALUMNA VS. ALUMNAE

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.

Quiz yourself on alum vs. alumnus!

True or False? 

Alum and alumnus are both always used in a gender-neutral way.

How to use alum in a sentence

British Dictionary definitions for alum

alum
/ (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
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

Word Origin for alum

C14: from Old French, from Latin alūmen
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for alum

alum
[ ăləm ]

n.
Any of various double sulfates of a trivalent metal such as aluminum or iron and a univalent metal such as potassium or sodium that are used as topical astringents and styptics.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for alum

alum
[ ăləm ]

Any of various crystalline double salts of a trivalent metal (such as aluminum, chromium, or iron) and a monovalent metal (such as potassium or sodium), especially aluminum potassium sulfate. Alum is widely used in industry as a hardener and purifier, and in medicine as an emetic and to stop bleeding.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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