To notice or become aware of something; to realize or perceive a fact or change.
出発してから財布を忘れたことに気づいた。
I realized I'd forgotten my wallet after leaving.
彼女の表情の変化に誰も気づかなかった。
No one noticed the change in her expression.
その説明を聞いて初めて問題の重要さに気づいた。
Only after hearing that explanation did I realize how important the issue was.
彼は周囲の空気にすぐ気づくタイプだ。
He's the type who quickly notices the atmosphere around him.
To regain consciousness; to come to (one's senses) after fainting or being unconscious.
救急隊が到着してしばらくして彼は気づいた。
He came to a little while after the paramedics arrived.
頭を打った後、ゆっくりと気づいたので病院へ運ばれた。
After hitting his head he slowly regained consciousness and was taken to hospital.