to notice; to realize; to realise; to become aware (of); to perceive; to sense; to suspect
to be attentive; to be scrupulous; to be observant
to regain consciousness; to come to (one's senses); to come round
財布がないことに気がついた。
I noticed my wallet was missing.
誰も間違いに気がつかなかった。
Nobody noticed the mistake.
朝起きたら雪が積もっていることに気がつく。
When I woke up in the morning, I noticed snow had piled up.