The Collaborative International Dictionary of English v.0.48:

Hilarity \Hi*lar"i*ty\ (?; 277), n. [L. hilaritas: cf. F.
   hilarit['e]. See Hilarious.]
   Boisterous mirth; merriment; jollity. --Goldsmith.
   [1913 Webster]

   Note: Hilarity differs from joy: the latter, excited by good
         news or prosperity, is an affection of the mind; the
         former, produced by social pleasure, drinking, etc.,
         which rouse the animal spirits, is more demonstrative.

   Syn: Glee; cheerfulness; mirth; merriment; gayety;
        joyousness; exhilaration; joviality; jollity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hilarity
    n 1: great merriment [syn: hilarity, mirth, mirthfulness,
         glee, gleefulness]