The Collaborative International Dictionary of English v.0.48:

Song \Song\ (s[o^]ng; 115), n. [AS. song, sang, fr. singan to
   sing; akin to D. zang, G. sang, Icel. s["o]ngr, Goth. saggws.
   See Sing.]
   1. That which is sung or uttered with musical modulations of
      the voice, whether of a human being or of a bird, insect,
      etc. "That most ethereal of all sounds, the song of
      crickets." --Hawthorne.
      [1913 Webster]

   2. A lyrical poem adapted to vocal music; a ballad.
      [1913 Webster]

   3. More generally, any poetical strain; a poem.
      [1913 Webster]

            The bard that first adorned our native tongue
            Tuned to his British lyre this ancient song.
                                                  --Dryden.
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   4. Poetical composition; poetry; verse.
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            This subject for heroic song.         --Milton.
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   5. An object of derision; a laughingstock.
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            And now am I their song, yea, I am their byword.
                                                  --Job xxx. 9.
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   6. A trifle; an insignificant sum of money; as, he bought it
      for a song. "The soldier's pay is a song." --Silliman.
      [1913 Webster +PJC]

   Old song, a trifle; nothing of value. "I do not intend to
      be thus put off with an old song." --Dr. H. More.

   Song bird (Zool.), any singing bird; one of the Oscines.


   Song sparrow (Zool.), a very common North American sparrow
      (Melospiza fasciata, or Melospiza melodia) noted for
      the sweetness of its song in early spring. Its breast is
      covered with dusky brown streaks which form a blotch in
      the center.

   Song thrush (Zool.), a common European thrush (Turdus
      musicus), noted for its melodius song; -- called also
      mavis, throstle, and thrasher.
      [1913 Webster]

   Syn: Sonnet; ballad; canticle; carol; canzonet; ditty; hymn;
        descant; lay; strain; poesy; verse.
        [1913 Webster]

WordNet (r) 3.0 (2006):

song
    n 1: a short musical composition with words; "a successful
         musical must have at least three good songs" [syn: song,
         vocal]
    2: a distinctive or characteristic sound; "the song of bullets
       was in the air"; "the song of the wind"; "the wheels sang
       their song as the train rocketed ahead"
    3: the act of singing; "with a shout and a song they marched up
       to the gates" [syn: song, strain]
    4: the characteristic sound produced by a bird; "a bird will not
       learn its song unless it hears it at an early age" [syn:
       birdcall, call, birdsong, song]
    5: a very small sum; "he bought it for a song"
    6: the imperial dynasty of China from 960 to 1279; noted for art
       and literature and philosophy [syn: Sung, Sung dynasty,
       Song, Song dynasty]

Moby Thesaurus II by Grady Ward, 1.0:

176 Moby Thesaurus words for "song":
   Brautlied, Christmas carol, English sonnet, Horatian ode,
   Italian sonnet, Kunstlied, Liebeslied, Petrarchan sonnet,
   Pindaric ode, Sapphic ode, Shakespearean sonnet, Volkslied, ado,
   air, alba, anacreontic, anthem, aria, art song, aubade, balada,
   ballad, ballade, ballata, barcarole, bel canto, blues, blues song,
   boat song, bother, bravura, bridal hymn, brindisi, bucolic,
   calypso, canso, canticle, canto, cantus, canzone, canzonet,
   canzonetta, carol, cavatina, chanson, chant, chantey, cheaply,
   choral singing, clerihew, coloratura, commotion, croon, croon song,
   crooning, cry, descant, dirge, dithyramb, ditty, drinking song,
   eclogue, elegy, epic, epigram, epithalamium, epode, epopee,
   epopoeia, epos, evasion, flap, folk singing, folk song, for a song,
   fuss, georgic, ghazel, haiku, hum, humming, hymeneal, hymn, idyll,
   inexpensively, intonation, jingle, lay, lied, lilt, limerick, line,
   love song, love-lilt, lyric, lyricism, madrigal, matin, measure,
   melodia, melodic line, melody, minstrel song, minstrelsy, monody,
   musical thought, narrative poem, national anthem, note, number,
   nursery rhyme, ode, operatic singing, palinode, pastoral,
   pastoral elegy, pastorela, pastourelle, performance, piece, poem,
   poesy, poetry, prevarication, prothalamium, refrain, rhyme,
   rondeau, rondel, roundel, roundelay, satire, scat, scat singing,
   serena, serenade, serenata, sestina, singing, sloka, sol-fa,
   sol-fa exercise, solfeggio, solmization, solo, solo part, sonnet,
   sonnet sequence, soprano part, strain, tale, tanka, tenso, tenzone,
   the supreme fiction, theme song, threnody, to-do, tonic sol-fa,
   torch song, treble, triolet, troubadour poem, tune, verse,
   verselet, versicle, villanelle, virelay, vocal music, vocalization,
   war song, warbling, wedding song, yodel, yodeling