(n.) 1. A key that opens every one of a given set of locks. Also called passkey.
(n.) 2. Key that secures entrance everywhere.

Source: Dictionary.com

(n.) 1. A key which operates all the master keyed locks or cylinders in a group, each lock or cylinder usually operated by its own change key.
(v.) 2. To combinate a group of locks or cylinders such that each is operated by its own change key as well as by a master key for the entire group.
(n.) 3. An automotive key which operates all or most locks on a vehicle where there is also a valet key employed.

Source: Lock Industry Standards and Training (LIST) Council

See also: “pass key”