1.  alum(n = noun.substance) ammonia alum, ammonium alum - a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum;
is a kind of al, aluminium, aluminum, atomic number 13, double salt

2.  alum(n = noun.substance) potash alum, potassium alum - a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum;
is a kind of al, aluminium, aluminum, atomic number 13, double salt

3.  alum(n = noun.person) alumna, alumnus, grad, graduate - a person who has received a degree from a school (high school or college or university);
is a kind of bookman, scholar, scholarly person, student
has particulars: ivy leaguer, old boy

4.  alum(n = noun.artifact) - a double sulphate of aluminum and potassium that is used as an astringent (among other things);
is a kind of astringent, astringent drug, styptic

   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]
" 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]

   To steep in, or otherwise impregnate with, a solution of alum; to treat with alum. Ure. [1913 Webster]

alum, n.
1 a double sulphate of aluminium and potassium.
2 any of a group of compounds of double sulphates of a monovalent metal (or group) and a trivalent metal.

Etymology:
ME f. OF f. L alumen aluminis

N  sourness, acid, acidity, low pH, acetous fermentation, lactic fermentation, vinegar, verjuice, crab, alum, acetic acid, lactic acid, sour, acid, acidulous, acidulated, tart, crabbed, acetous, acetose, acerb, acetic, sour as vinegar, sourish, acescent, subacid, styptic, hard, rough, sour as a lemon, (4) Odor.

alum root, alum schist, alum shale, alum stone, ammonia alum, ammonium alum, chrome alum, potash alum, potassium alum, roche alum