[count] 1 a : a device that is used to open a lock or start an automobile ◊The usual type of key is a small metal object that you insert into a narrow opening and turn. ▪ You need the key to get into the drawer. ▪ She turned the key and opened the door. ▪ house/car keys ▪ the ignition key of a car ▪ the key to the lock —often used figuratively ▪ That woman holds the key to his heart. —see also latchkey, passkey, skeleton key b : a device that looks like a key and that is used to turn something ▪ a key for winding a clock
2 : something that is necessary in order to do or achieve something —usually singular ▪ If you want to improve your health, exercise is the key. —usually + to ▪ The key to hitting the ball well is following through on your stroke. ▪ Hard work is the key to success.
3 a : something that provides an explanation or solution —usually + to ▪ the key to a riddle b : a list of words or phrases that explain the meaning of symbols or abbreviations ▪ Use the key to decode the symbols. ▪ a pronunciation key
4 a : any one of the buttons of a computer or typewriter that you push with your fingers ▪ He tapped away at the keys, typing his letter. —see picture at computer; see also function key, shift key b : any one of the parts that you push with your fingers to play a piano or similar musical instrument ▪ She sat down at the piano and put her hands on the keys.
5 music : a system of musical tones based on a scale beginning on the note for which the system is named ▪ the key of C ▪ The symphony changes key from G major to E minor.