Definition
to respond to a person's correspondence
回信
I sent her an email yesterday and hope she will write back soon.
我昨天给她发了一封电子邮件，希望她能尽快回复。
In this letter, I requested him to write back to confirm the meeting time.
在这封信中，我要求他回信确认会议时间。
If you have any questions, please feel free to write back, and I will respond as soon as possible.
如果你有任何问题，请随时回信，我会尽快回复。