The Collaborative International Dictionary of English v.0.48:

Alum \Al"um\, n. [OE. alum, alom, OF. alum, F. alun, fr. L.
   alumen alum.] (Chem.)
   A double sulphate formed of aluminium and some other element
   (esp. an alkali metal) or of aluminium. It has twenty-four
   molecules of water of crystallization.
   [1913 Webster]

   Note: Common alum is the double sulphate of aluminium and
         potassium. It is white, transparent, very astringent,
         and crystallizes easily in octahedrons. The term is
         extended so as to include other double sulphates
         similar to alum in formula.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Alum \Al"um\, v. t.
   To steep in, or otherwise impregnate with, a solution of
   alum; to treat with alum. --Ure.
   [1913 Webster]

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)