The Collaborative International Dictionary of English v.0.48:

Ditty \Dit"ty\, n.; pl. Ditties. [OE. dite, OF. diti['e], fr.
   L. dictatum, p. p. neut. of dictare to say often, dictate,
   compose. See Dictate, v. t.]
   1. A saying or utterance; especially, one that is short and
      frequently repeated; a theme.
      [1913 Webster]

            O, too high ditty for my simple rhyme. --Spenser.
      [1913 Webster]

   2. A song; a lay; a little poem intended to be sung.
      "Religious, martial, or civil ditties." --Milton.
      [1913 Webster]

            And to the warbling lute soft ditties sing.
                                                  --Sandys.
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The Collaborative International Dictionary of English v.0.48:

Ditty \Dit"ty\, v. i.
   To sing; to warble a little tune.
   [1913 Webster]

         Beasts fain would sing; birds ditty to their notes.
                                                  --Herbert.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ditty
    n 1: a short simple song (or the words of a poem intended to be
         sung)

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "ditty":
   Brautlied, Christmas carol, Kunstlied, Liebeslied, Volkslied, alba,
   ana, anthem, art song, aubade, ballad, ballade, ballata, barcarole,
   blues, blues song, boat song, bridal hymn, brindisi, calypso,
   canso, canticle, canzone, canzonet, canzonetta, carol, cavatina,
   chanson, chant, chantey, croon, croon song, descant, dirge,
   drinking song, epithalamium, folk song, hymeneal, hymn, lay, lied,
   lilt, love song, love-lilt, matin, minstrel song, minstrelsy,
   national anthem, prothalamium, serena, serenade, serenata, song,
   theme song, torch song, war song, wedding song