From Longman Dictionary of Contemporary Englishmaster keyˈmaster key noun [countable] DHHa key that will open all the door locks in a building
Examples from the Corpus
master key• I went for help but the porter's master key also failed.• Safe in her own hallway, Miss Worthington bolted the front door, turned the master key.• The master key both advances the stage and records the total number of points counted. 