[al.um] n [ME, fr. MF alum, alun, fr. L alumen] (14c) 1: a potassium aluminum sulfate or an ammonium aluminum sulfate used esp. as an emetic and as an astringent and styptic
2: any of various double salts isomorphous with potassium aluminum sulfate
3: aluminum sulfate
[2]alum n [by shortening] (1930): alumnus, alumna