Definition of Alum

Noun: alum a-lum

A white crystalline double sulphate of aluminium: the ammonium double sulphate of aluminium
- ammonia alum, ammonium alum
A white crystalline double sulphate of aluminium: the potassium double sulphate of aluminium
- potassium alum, potash alum
A person who has received a degree from a school (high school, college or university)
- alumnus, alumna, graduate, grad
A double sulphate of aluminium and potassium that is used as an astringent (among other things)