Wordnet / HyperDictionary Definition

1. The perception of pleasant arrangements of musical notes
2. A succession of notes forminga distinctive sequence; "she was humming an air from Beethoven"

Wikipedia Definition

In music, a melody is a series of linear events or a succession, not a simultaneity as in a chord. However, this succession must contain change of some kind and be perceived as a single entity (possibly gestalt) to be called a melody. Most specifically this includes patterns of changing pitches and durations, while most generally it includes any interacting patterns of changing events or quality. Melody may be said to result where there are interacting patterns of changing events occurring in time.

"The events occurring in time must involve change of some kind to be understood as related or unrelated...The essential elements of any melody are duration, pitch, and quality [timbre, texture, and loudness]" Melodies often consist of one or more musical phrases, motifs, and is usually repeated throughout a song or piece in various forms. Melodies may also be described by their melodic motion or the pitches or the intervals between pitches (predominately conjuct or disjunct or with further restrictions), pitch range, tension and release, continuity and coherence, cadence, and shape.