In tune; striking or singing the proper notes. Opposite of <ant>off-key</ant>.(adjective)
Gnu Collaboartive International Dictionary of English: licensed under The Code Project Open License (CPOL)
Use "on-key" in a sentence
"Model Karolina Kurkova, who presented an award while wearing a hooded DVF sequined jumpsuit, began singing in a surprisingly on-key voice, the lyrics to Lady Gaga's song "Paparazzi," before stopping herself."
" He had this big car in the dream, a snazzy Mark IV, all midnight purple like Batman's, with bitchin 'chrome rims, cruise control, and a sound system made for the angels who scream out to heaven and hell, and on-key."
"Yet still, muffled under, that old sweet song croaked on-key."