1Writing (the act of writing text or notes on paper or in a notebook), or the written entries themselves.
教科書の余白に鉛筆で書き込みをした。
I wrote notes in the margins of the textbook with a pencil.
このノートは過去の書き込みが多くて見返すと参考になる。
This notebook has many past notes, which are useful to look back on.
重要なところに赤で書き込みを入れた。
I added notes in red where it was important.
2Filling out an entry on a form; an entry on a form or application (i.e., the act or field of writing required information).
申込書の書き込み欄に住所と電話番号を記入してください。
Please fill in your address and phone number in the application form's entry fields.
書き込みミスがあったので訂正印が必要だ。
There was a mistake in the entry, so a correction stamp is required.
3Posting to an online forum, blog, or SNS; a post or comment made online.
掲示板に書き込みをしたら、すぐに返信が来た。
I posted on the message board and got a reply immediately.
その動画のコメント欄の書き込みが炎上している。
The comments on that video are getting heated / the comment section is flaming.
書き込みを削除するにはログインが必要だ。
You need to log in to delete a post.