Master key
The root key of a key hierarchy, rarely used for encryption directly.
The master key sits at the top of a key hierarchy and is used to derive or wrap other keys. Ideally it never leaves its HSM in plaintext, and whoever can reach it effectively has access to everything beneath it. For that reason it is used sparingly, and seldom to encrypt bulk data directly.
Rotating a master key is a much bigger job than rotating a data key, because everything wrapped or derived under it must be re-encrypted or re-derived. Many systems therefore keep it offline and use it only in planned key ceremonies.