Here's what the OED has:
a. A combination of two 'according' or harmonious notes sounded together, a CONCORD. Obs.
a. A combination, concordant or discordant, of three or more simultaneous notes according to the rules of harmony; rarely of two notes only.
common chord (also perfect chord): the combination of any note with its third (major or minor), perfect fifth, and octave. Chord is often used alone for common chord, e.g. 'the chord of C'.
So what would 2 notes sounding at the same time be?