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.
A key that can open multiple locks; less common than the duplicate key sense.
管理人は合鍵で全ての部屋に入れる。
The caretaker can enter all rooms with a master key.
Standard kanji spelling.
Variant spelling with okurigana.
Mixed kanji-katakana spelling; uncommon.
Compound of 合い (ai, 'together, matching') + 鍵 (kagi, 'key').