master key

WordReference Random House Learner's Dictionary of American English © 2025

ˈmas•ter ˌkey, n. [countable]

1. Building a key that will open a number of different locks.

WordReference Random House Unabridged Dictionary of American English © 2025

mas′ter key′, 

1. Building a key that will open a number of different locks, the proper keys of which are not interchangeable.

• 1570–80

Collins Concise English Dictionary © HarperCollins Publishers::

master key noun

1. a key that opens all the locks of a set, the individual keys of which are not interchangeable
   Also called: pass key