key

1 [kee] 

noun, plural keys.

1.  A small metal instrument specially cut to fit into a lock and move its bolt.
2.  Any of various devices resembling or functioning as a key: the key of a clock.
3.  Key card.
4.  Something that affords a means of access: the key to happiness.
5.  Something that secures or controls entrance to a place: Gibraltar is the key to the Mediterranean.
adjective
6.  Chief; major; important; essential; fundamental; pivotal: a key person in the company; key industries.