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]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "hilarity":
   Homeric laughter, absurdity, absurdness, amusement, belly laugh,
   bizarreness, bizarrerie, boff, boffola, boisterousness,
   burst of laughter, cachinnation, cackle, cheerfulness, chortle,
   chuckle, conviviality, convulsion, crow, drollery, drollness,
   eccentricity, elation, exhilaration, exuberance, fit of laughter,
   frivolity, fun, funniness, gaiety, gales of laughter, giggle, glee,
   gleefulness, guffaw, ha-ha, hearty laugh, hee-haw, hee-hee,
   high glee, high spirits, hilariousness, ho-ho, horselaugh,
   humorousness, incongruity, jocularity, jocundity, jolliness,
   jollity, joviality, joy, joyfulness, joyousness, jubilation, laugh,
   laughability, laughing, laughter, levity, ludicrousness, merriment,
   merriness, mirth, mirthfulness, outburst of laughter,
   peal of laughter, pricelessness, quaintness, quizzicalness,
   revelry, richness, ridiculousness, risibility, roar of laughter,
   shout, shout of laughter, shriek, snicker, snigger, snort, tee-hee,
   the funny side, titter, vivacity, whimsicalness, wittiness, yuck,
   yuk-yuk