返事 means "reply," "answer," or "response." It refers to what you say or write back to someone after they've spoken or written to you. You often use it with "する" (返事する). For example, "メールの返事を書く" means "to write a reply to an email."

質問や連絡などに対して答えることです。「返事を待つ」のように使います。