From WordNet (r) 3.0 (2006):
alum
n 1: a white crystalline double sulfate of aluminum: the
ammonium double sulfate of aluminum [syn: alum, ammonia
alum, ammonium alum]
2: a white crystalline double sulfate of aluminum: the potassium
double sulfate of aluminum [syn: alum, potassium alum,
potash alum]
3: a person who has received a degree from a school (high school
or college or university) [syn: alumnus, alumna, alum,
graduate, grad]
4: a double sulphate of aluminum and potassium that is used as
an astringent (among other things)