master key 
noun
A key that opens several locks, each of which also has its own key: the master key for all the safes in the department...
- The change key will open only that specific lock, while the master key will open that lock and several others in a group.
- She used her master key to open the lock, and began to open the gate.
- I got the master key and opened the door and that's when I saw him in the bath.