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
Now, the Johnson & Wales alum is focused on planning his next steps. — Tanasia Kenney, Charlotte Observer, 29 Oct. 2026 Most of his fellow cast members were Americans who had worked in TV, like Night Court alum Brent Spiner. — Jordan Hoffman, Vanity Fair, 29 Oct. 2026 When the season 35 cast was announced in September, the Food Network alum told PEOPLE Jade had inspired her to do the show. — Erin Clements, PEOPLE, 29 Oct. 2026 From 2005 to 2014, former The Daily Show alum played a larger-than-life, egotistical caricature of himself for Comedy Central’s The Colbert Report. — Andrew Paul, Popular Science, 28 Oct. 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

Browse Nearby Words

Cite this Entry

“Alum.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/alum. Accessed 31 Oct. 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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