melody

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A pleasing succession or arrangement of sounds.
• noun Musical quality.
• noun A rhythmically organized sequence of single tones so related to one another as to make up a particular phrase or idea.
• noun Structure with respect to the arrangement of single notes in succession.
• noun The leading part or the air in a composition with accompaniment.
• noun A poem suitable for setting to music or singing.

from The Century Dictionary.

• noun In general, a succession of agreeable musical sounds; sweet sound; song; tune; music.
• noun Specifically In music: A succession of tones, whether pleasing or not.
• noun The underlying relationship may be variously established: by any particular rhythmic arrangement, as in some popular dance-tunes; by the intervals of a single chord, as in arpeggio phrases; by a diatonic order, as in scale passages; by the harmonic connections between successive chords of which the melody in question forms one of the voice-parts, as in simple choral writing; and by innumerable modifications and combinations of these and similar principles.
• noun A melody is authentic when its compass extends about an octave upward from its key-note or final, plagal when its compass extends about a half-octave above and below the key-note and final. It is diatonic when it uses only the proper tones of the scale in which it is written, chromatic when it uses other tones, foreign to that scale. It is concrete or conjunct when it proceeds by single degrees, upward or downward; discrete or disjunct when it proceeds by steps of more than a single degree. It is syllabic when but one tone is given to each syllable of the words; slurred when more than one tone is given to a syllable. A melody may be further described as popular, national, artistic, etc.
• noun A melodious or tuneful poem; a poetical composition suitable for singing.

from the GNU version of the Collaborative International Dictionary of English.

• noun A sweet or agreeable succession of sounds.
• noun (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.
• noun The air or tune of a musical piece.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun tune; sequence of notes that makes up a musical phrase

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a succession of notes forming a distinctive sequence
• noun the perception of pleasant arrangements of musical notes