Many hotels now use magnetic card keys that don't need to be carried physically.

magnetic 来自名词 magnet（磁铁），加上形容词后缀 -ic 构成形容词，意思是"磁性的、有磁力的"。 card keys 指"卡片钥匙"，magnetic card keys 特指利用磁性原理工作的"磁卡钥匙"。