(idiomatic) to hold the key (to)

(Internet slang) protected account (on social media, especially Twitter and Instagram)

to lock (a door, etc.) / (Internet slang) (of a SNS account) to change privacy setting to limited people and posts and/or profiles can be seen by limited people.

black key (of a piano, etc.)

white key (of a piano, etc.)

(cryptography) secret key

(cryptography) public key

(cryptography) key exchange - an exchange of peers' keys at initialization phase of connection.

representing relationship to a family

house of something, place where some business is conducted / person who does that thing