What does KEY mean?

noun

1. metal device shaped in such a way that when it is inserted into the appropriate lock the lock's mechanism can be rotated
2. something crucial for explaining“the key to development is economic integration”
3. pitch of the voice“he spoke in a low key”
4. any of 24 major or minor diatonic scales that provide the tonal framework for a piece of music
5. a kilogram of a narcotic drug“they were carrying two keys of heroin”
6. a winged often one-seed indehiscent fruit as of the ash or elm or maple
7. a coral reef off the southern coast of Florida
8. (basketball) a space (including the foul line) in front of the basket at each end of a basketball court; usually painted a different color from the rest of the court“he hit a jump shot from the top of the key”