Being in the state of working or in the middle of a task; often used on notices to indicate work is in progress.
ただいまサーバーのメンテナンス作業中です。しばらくお待ちください。
The server is currently under maintenance work. Please wait a moment.
入室禁止：作業中のため立ち入りを禁じます。
No entry: Do not enter due to work in progress.
彼は新しいプロジェクトの資料作りで今週ずっと作業中だ。
He has been working all week preparing materials for a new project.