The Collaborative International Dictionary of English v.0.48:

Melody \Mel"o*dy\, n.; pl. Melodies. [OE. melodie, F.
   m['e]lodie, L. melodia, fr. Gr. ? a singing, choral song, fr.
   ? musical, melodious; me`los song, tune + ? song. See Ode.]
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   1. A sweet or agreeable succession of sounds.
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            Lulled with sound of sweetest melody. --Shak.
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   2. (Mus.) A rhythmical succession of single tones, ranging
      for the most part within a given key, and so related
      together as to form a musical whole, having the unity of
      what is technically called a musical thought, at once
      pleasing to the ear and characteristic in expression.
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   Note: Melody consists in a succession of single tones;
         harmony is a consonance or agreement of tones, also a
         succession of consonant musical combinations or chords.
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   3. The air or tune of a musical piece.
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   Syn: See Harmony.
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WordNet (r) 3.0 (2006):

melody
    n 1: a succession of notes forming a distinctive sequence; "she
         was humming an air from Beethoven" [syn: tune, melody,
         air, strain, melodic line, line, melodic phrase]
    2: the perception of pleasant arrangements of musical notes
       [syn: melody, tonal pattern]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "melody":
   air, aria, bel canto, canto, cantus, descant, diapason, dulcetness,
   euphoniousness, euphony, harmony, lay, line, lyrics, measure,
   mellifluence, mellifluousness, melodia, melodic line,
   melodiousness, musical quality, musical sound, musicality, note,
   refrain, solo, solo part, song, soprano part, strain, sweetness,
   theme, tonality, tone, treble, tune, tunefulness, warble