The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         A group of tittering pages ran before.   --Longfellow.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Titter \Tit"ter\, n.
   A restrained laugh. "There was a titter of . . . delight on
   his countenance." --Coleridge.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Titter \Tit"ter\, v. i.
   To seesaw. See Teeter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

titter
    n 1: a nervous restrained laugh
    v 1: laugh nervously; "The girls giggled when the rock star came
         into the classroom" [syn: giggle, titter]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "titter":
   Homeric laughter, be in stitches, belly laugh, boff, boffola,
   break up, burst into laughter, burst of laughter, burst out,
   burst out laughing, burst with laughter, bust a gut, cachinnate,
   cachinnation, cackle, chortle, chuckle, convulsion, crow,
   fit of laughter, gales of laughter, giggle, go into convulsions,
   guffaw, ha-ha, hearty laugh, hee-haw, hee-hee, hilarity, ho-ho,
   horselaugh, laugh, laugh it up, laugh outright, laughing, laughter,
   nearly die laughing, outburst of laughter, peal of laughter,
   risibility, roar, roar of laughter, roar with laughter,
   shake like jelly, shake with laughter, shout, shout of laughter,
   shriek, snicker, snigger, sniggle, snort, split,
   split with laughter, tee-hee, twitter, yuck, yuk-yuk