key
lock
key (to a problem, success, etc.); clue
「鍵」 has another reading with different meanings.
key (on a piano, organ, etc.)
lock; padlock
鍵をどこに置いたか忘れた。
I forgot where I put the key.
出かけるときは必ず鍵をかけてね。
Make sure to lock the door when you go out.
スペアの鍵を作っておいたほうがいいよ。
You should have a spare key made.