Definition of "Alum" :

noun: a double sulphate of aluminum and potassium that is used as an astringent (among other things)

noun: a person who has received a degree from a school (high school or college or university)

noun: a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum

noun: a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum