What is the meaning of the word "key"?

Definitions:

a generic term for any device whose possession entitles the holder to a means of access noun
- a safedeposit box usually requires two keys to open it
a kilogram of a narcotic drug noun
- they were carrying two keys of heroin
a list of answers to a test noun
- some students had stolen the key to the final exam
harmonize with or adjust to verb
- key one's actions to the voters' prevailing attitude
pitch of the voice noun
- They spoke in a low key.
provide with a key verb
- We were keyed after the locks were changed in the building
something crucial for explaining noun
- the key to development is economic integration
vandalize a car by scratching the sides with a key verb
- My new Mercedes was keyed last night in the parking lot.