noun
duplicate key; copy of a key
A key made as a copy of an original, often without the owner's knowledge.
彼女はこっそり合鍵を作った。
She secretly made a duplicate key.
noun
master key; passkey; skeleton key
A key that can open multiple locks; less common than the duplicate key sense.
The caretaker can enter all rooms with a master key.
General word for key or lock; 合鍵 specifically refers to a duplicate or master key.
Compound of 合い (ai, 'together, matching') + 鍵 (kagi, 'key').