A chord was several musical notes played at the same time.
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")