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]