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]