.....Key.....
On its own,
Key is usually thought to mean a key which has been tooled specifically to lock or unlock any door. Or
Key could be used in terms of implying keys to operate and drive a car.
Key is sometimes expressed as something of critical importance (i.e., key witness, key factors, key to the investigation, key concept, key components, key ideas, someone holds the key to your heart..... and so on).
