written response; reply in writing
Formal term for a written answer, often used in legal, governmental, or official correspondence.
I submitted a written response to the court.
Please check the contents of the written reply.
回答 is a broader term for an answer or response, not necessarily in writing, and can be used in everyday contexts.
Compound of 答弁 (answer, defense) + 書 (document). The exact historical derivation is uncertain, but the word is conventionally associated with formal written replies.