The blending or fusing of the two notes in an interval or the many notes in a chord into a sound that seems to be 'at rest'.
It is one pole of the continuum of musical accordance, the other pole being that of discordance.
A conglomeration of pitches can be said to be a concord based strictly on psychoacoustic phenomena, and regardless of any use of that particular conglomeration in a musical context. For the latter, see sonance.
The term is also important in the concept of "tuning by concords", which see.