[ al-uhm ]
See synonyms for alum on
  1. 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.

  2. 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.

  1. (not in technical use) aluminum sulfate.

Origin of alum

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 )

Words Nearby alum

Other definitions for alum (2 of 3)

[ uh-luhm ]

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

Origin of alum

First recorded in 1875–80; by shortening

usage note For alum

What's the difference between alum, alumnus, and alumna? See alumnus.

Words that may be confused with alum

Other definitions for alum. (3 of 3)


  1. aluminum. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use alum in a sentence

British Dictionary definitions for alum


/ (ˈæləm) /

  1. 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

  2. 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

Origin of 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

Scientific definitions for alum


[ ăləm ]

  1. 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.