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noun (1)

al·​um ˈa-ləm How to pronounce alum (audio)
: 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
: any of various double salts isomorphous with potassium aluminum sulfate


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noun (2)

: alumnus, alumna
a Yale alum


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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
Later that month, the Dawson’s Creek alum showed off her baby bump at the Cannes Film Festival. Ariana Quihuiz, Peoplemag, 10 June 2024 Rob Lowe shares Brat Pack throwback photo for St. Elmo's Fire anniversary Fellow Brat Pack alum Moore — who wasn’t in the film but starred opposite McCarthy in St. Elmo’s Fire — offered the sole dissenting opinion. Shania Russell,, 9 June 2024 Profiling as a yards-after-catch monster is Saints slot receiver Rashid Shaheed, a Mt. Carmel High School alum and one of the NFL’s better returners. Tom Krasovic, San Diego Union-Tribune, 9 June 2024 Others in the new season include four previous Survivor contestants, Big Brother alums Britney Haynes and Danielle Reyes, The Real Housewives franchise faces like Chanel Ayan and Dolores Catania, Vanderpump Rules star Tom Sandoval, and former RuPaul’s Drag Race winner Bob the Drag Queen. Rebecca Schneid, TIME, 7 June 2024 See all Example Sentences for alum 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'alum.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


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

Dictionary Entries Near alum

Cite this Entry

“Alum.” Dictionary, Merriam-Webster, Accessed 20 Jun. 2024.

Kids Definition


: either of two colorless crystalline compounds containing aluminum that are used in medicine (as to check local sweating or to stop bleeding)

Medical Definition


al·​um ˈal-əm How to pronounce alum (audio)
: either of two colorless or white crystalline double sulfates of aluminum used in medicine internally as emetics and locally as astringents and styptics:
: one KAl(SO4)2·12H2O that is a sulfate of aluminum and potassium

called also potassium alum

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

called also ammonia alum, ammonium alum

: any of various double salts isomorphous with potassium aluminum sulfate

More from Merriam-Webster on alum

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