The Musical Definition of Melody

Melody is the main tune of a song; the outcome of a series of notes. Melody is regarded as “horizontal” because its notes are read from left-to-right, while harmony is “vertical” because the notes are played simultaneously (and therefore must be written vertically in notation).

The complexity of a song is observed in its texture. Musical texture can be simple or elaborate - and everything in between - and melody fits into this concept in the following ways:

Monophonic: A single line of melody with no harmony.
Biphonic: Two different, simultaneous melodies (though some overlap of notes may create harmonies).
Heterophonic: A slightly elaborated melody with one or two voices and some intervals. Harmony is sprinkled throughout.
Homophonic: A single line of melody with harmony in the form of chords.
Polyphonic: A composition with many voices and harmonies. Many melodies may appear throughout the composition.