There are 2 definitions of alum in English:

alum1

Line breaks: alum
Pronunciation: /ˈaləm
 
/
(also potash alum)

noun

[mass noun] Chemistry
1A colourless astringent compound which is a hydrated double sulphate of aluminium and potassium, used in solution in dyeing and tanning.
  • Chemical formula: AlK(SO4)2.12H2O
More example sentences
  • It could also be treated by rubbing salt, brain or potash alum into the surface to produce a very pale leather.
  • Astringent varieties contain alum, which makes your mouth pucker when the fruits are eaten before they're fully ripe.
  • I have no idea how it's manufactured, but it's a kind of alum, a double sulfate of aluminum in crystal salt form.
1.1 [count noun] Any of a number of analogous crystalline double sulphates of a monovalent metal (or group) and a trivalent metal: artificially grown crystals of one of the alums
More example sentences
  • Several decades later the Englishman Davy attempted to obtain the metal hidden in alums.
  • Aluminium dross tailings were used to produce two types of alums.

Origin

late Middle English: via Old French from Latin alumen, alumin- related to aluta 'tawed leather'.

Definition of alum in:

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Word of the day envenom
Pronunciation: enˈvenəm
verb
put poison on or into; make poisonous

There are 2 definitions of alum in English:

alum2

Syllabification: a·lum
Pronunciation: /əˈləm/

Entry from US English dictionary

noun

informal
An alumnus or alumna: a fellow Wellesley alum

Origin

early 20th century: abbreviation.

Definition of alum in: