alum

1 of 3

noun (1)

al·​um ˈa-ləm 

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

: aluminum sulfate

alum

2 of 3

noun (2)

ə-ˈləm 

: alumnus, alumna

a Yale alum

alum

3 of 3

abbreviation

aluminum

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

Noun

Turns out, Stossel was a Princeton alum and a donor to Princeton in Africa. —Literary Hub, 6 Feb. 2026

Less than a year ago (in March 2025, before Prada announced its intentions to buy the label), Versace named Dario Vitale – himself a Prada alum, having worked for its brainy brand Miu Miu for over a decade – as its creative director after Donatella Versace stepped aside. —Rachel Tashjian, CNN Money, 5 Feb. 2026

The move was ridiculed by lawyers outside Paul Weiss, and more than 140 alums of the firm signed a letter assailing it as well. —Meg Kinnard, Los Angeles Times, 5 Feb. 2026

Season 2's 18th episode will also feature other Young Sheldon alums in guest roles, including Zoe Perry as Mary, Raegan Revord as Missy, Rex Linn as Principal Petersen, and Doc Farrow as Coach Wilkins. —Sydney Bucksbaum, Entertainment Weekly, 5 Feb. 2026

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

Rhymes for alum

• balam
• clallam
• kalam
• malum
• corallum
• paspalum

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

: aluminum sulfate

Medical Definition

alum

noun

al·​um ˈal-əm 

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