duplicate key; copy of a key
A key made as a copy of an original, often without the owner's knowledge.
She secretly made a duplicate key.
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.
Variant spelling with okurigana.
Mixed kanji-katakana spelling; uncommon.
Compound of 合い (ai, 'together, matching') + 鍵 (kagi, 'key').