In conventional musical parlance, the vertical chordal structure of music, in contrast to the horizontal melodic structure. The terms 'vertical' and 'horizontal' derive from western music notation where these graphic dimensions are used to prescribe events in PITCH and time, respectively. This division is analytic, as all sound happens in both dimensions (and others) at once. See: POLYPHONY.
In modern musical parlance, harmony is chordal process in music, i.e. pitch relations: among pitches in groups called chords, and among the chords. The manner in which the pitches are articulated is open, from simultaneously or 'vertically' to sequentially or 'horizontally'. In western pedagogy, harmony usually refers to a closed, systematic study of these relations in the music of 18th and 19th century Europe.
Colloquially, the term is used primarily as equivalent to concord and CONSONANCE, that is, expressing the pleasing combination of two or more sounds.