Source : Webster's Revised Unabridged Dictionary (1913)

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

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

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

   Syn: Merriment; joyousness; gladness; fun; frolic; glee;
        hilarity; festivity; jollity. See {Gladness}.

Source : WordNet®

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