noun
post office box; P.O. box
A private mailbox rented at a post office for receiving mail.
郵便私書箱を開設するには、郵便局で申し込みが必要です。
To open a post office box, you need to apply at the post office.
Compound of 郵便 (postal service), 私書 (private correspondence), and 箱 (box).