alum meaning and definition

ALUM noun

Definition of alum (noun)

1. a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum
   • synonyms: ammonia alum, ammonium alum
2. a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum
   • synonyms: potash alum, potassium alum
3. a person who has received a degree from a school (high school or college or university)
   • synonyms: alumna, alumnus, grad, graduate
4. a double sulphate of aluminum and potassium that is used as an astringent (among other things)