The Collaborative International Dictionary of English v.0.48:

Mirth \Mirth\, n. [OE. mirthe, murthe, merthe, AS. myr[eth],
   myrg[eth], merh[eth], mirh[eth]. See Merry.]
   [1913 Webster]
   1. Merriment; gayety accompanied with laughter; jollity.
      [1913 Webster]

            Then will I cause to cease . . . from the streets of
            Jerusalem, the voice of mirth.        --Jer. vii.
                                                  34.
      [1913 Webster]

   2. That which causes merriment. [Obs.] --Shak.
      [1913 Webster]

   Syn: Merriment; joyousness; gladness; fun; frolic; glee;
        hilarity; festivity; jollity. See Gladness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "mirth":
   amusement, buoyancy, cheer, cheerfulness, diversion, divertisement,
   divertissement, enjoyment, entertainment, frivolity, frolic, fun,
   gaiety, gladness, glee, gleefulness, happiness, high glee,
   high spirits, hilariousness, hilarity, jocularity, jocundity,
   jolliness, jollity, joviality, joy, joyfulness, joyousness,
   laughter, levity, lightheartedness, merriment, merriness,
   merrymaking, mirthfulness, passe-temps, pastime, pleasure,
   recreation, regalement, rejoicing, relaxation, revelry, solace