noun
A key that opens multiple locks, typically in a building or facility.
管理人はマスターキーで全ての部屋に入れる。
The caretaker can enter all rooms with the master key.
Variant with interpunct; less common in running text.
合鍵 is a duplicate key for a specific lock, while マスターキー opens many different locks.
Loanword from English 'master key'.