1
: a potassium aluminum sulfate KAl(SO4)2·12H2O or an ammonium aluminum sulfate NH4Al(SO4)2·12H2O used especially for its astringent and styptic properties
2
: any of various hydrated salts usually consisting of aluminum sulfate with a second cation
3
plural alums
informal
: alumnus, alumna
university students and alums
a Yale alum

alum

3 of 3

abbreviation

Did you know?

Is it acceptable to use alum for alumnus or alumna?

The words that we have used to refer to people who have attended or graduated from a school, college, or university have changed a bit over the past several centuries. Traditionally, the word alumnus has been used to refer to a single male, whereas alumna has been used for a single woman. Initially the plural forms were alumni to refer to multiple men (or multiple men and women) and alumnae for multiple women. A little over a hundred years ago the shortened form of alum began to be used to describe a graduate or past attendee of either gender. Although many people feel that alum is informal, it is in increasing use, and we appear to be moving toward a greater acceptance of the word. The plural of alum is alums.

Examples of alum in a Sentence

Recent Examples on the Web
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Noun
No Haskell alum has won the Travers since Keen Ice upset Triple Crown winner American Pharoah in 2015. Guy Martin, Forbes.com, 29 Aug. 2026 One Southridge alum even came over to join Hill, White and their friends during the game. Austin Horn august 29, Miami Herald, 29 Aug. 2026 Atwell, a Louisville alum, was also a free agent pickup in March who was expected to see playing time in a receiving corps that didn’t have many options before the draft and after trading away Jaylen Waddle to the Denver Broncos. David Furones, Sun Sentinel, 27 Aug. 2026 The Coach alum died of pneumonia, her family confirms to PEOPLE. Charna Flam, PEOPLE, 27 Aug. 2026 See All Example Sentences for alum

Word History

Etymology

Noun (1)

Middle English, from Anglo-French alum, alun, from Latin alumen

Noun (2)

by shortening

First Known Use

Noun (1)

14th century, in the meaning defined at sense 1

Noun (2)

1877, in the meaning defined above

Time Traveler
The first known use of alum was in the 14th century

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Cite this Entry

“Alum.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/alum. Accessed 30 Aug. 2026.

Kids Definition

alum

noun
al·​um
ˈal-əm
1
: either of two colorless crystalline compounds containing aluminum that are used in medicine (as to check local sweating or to stop bleeding)
2

Medical Definition

1
: either of two colorless or white crystalline double sulfates of aluminum used in medicine internally as emetics and locally as astringents and styptics:
a
: one KAl(SO4)2·12H2O that is a sulfate of aluminum and potassium

called also potassium alum

b
: one consisting of an ammonium aluminum sulfate NH4Al(SO4)2·12H2O

called also ammonia alum, ammonium alum

2
: any of various double salts isomorphous with potassium aluminum sulfate

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