The Collaborative International Dictionary of English v.0.48:
on-key \on-key\ adj. (Music) In tune; striking or singing the proper notes. Opposite of off-key. Syn: true. [WordNet 1.5]WordNet (r) 3.0 (2006):
on-key adj 1: in tune; accurate in pitch; "a true note" [syn: on-key, true]