Source : Webster's Revised Unabridged Dictionary (1913)

Mirthful \Mirth"ful\, a.
   1. Full of mirth or merriment; merry; as, mirthful children.

   2. Indicating or inspiring mirth; as, a mirthful face.

            Mirthful, comic shows.                --Shak.
      -- {Mirth"ful*ly}, adv. -- {Mirth"ful*ness}, n.

Source : WordNet®

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