A chord was several musical notes played in harmony.
While on Earth prior to 2151, Doctor Phlox's study of religions led him to spend two weeks at a Tibetan monastery learning to sing chords with the high lamas. (ENT: "Cold Front")
In 2364, Sonny Clemonds asked if he could borrow a guitar. When Data told him that the computer could replicate one for him, he was less than optimistic about how long it would take, saying, " I was kind of hoping to get one while I still remember the chords." (TNG: "The Neutral Zone")
In 2369, Jean-Luc Picard commented on Nella Daren's choice to play an F minor chord rather than a diminished D, saying it was (pleasantly) unexpected. (TNG: "Lessons")