Key

1. Metal device shaped in such a way that when it is inserted into the appropriate lock the lock's mechanism can be rotated. We need a key to start our car.
2. A similar device used to open or winding. Insert your key and wind up the clock.
3. A vital, crucial element. Timing was the key to making money on the stock market.
4. A tonal system consisting of seven tones in fixed relationship to a tonic, having a characteristic key signature. The song was in the key of C.