Source : Webster's Revised Unabridged Dictionary (1913)

Titter \Tit"ter\, v. i.
   To seesaw. See {Teeter}.

Titter \Tit"ter\, v. t. [imp. & p. p. {Tittered}; p. pr. & vb.
   n. {Tittering}.] [Probably of imitative origin.]
   To laugh with the tongue striking against the root of the
   upper teeth; to laugh with restraint, or without much noise;
   to giggle.

         A group of tittering pages ran before.   --Longfellow.

Titter \Tit"ter\, n.
   A restrained laugh. ``There was a titter of . . . delight on
   his countenance.'' --Coleridge.

Source : WordNet®

titter
     n : a nervous restrained laugh
     v : laugh nervously; "The girls giggled when the rock star came
         into the classroom" [syn: {giggle}]