box number

Etymology
Literally, the number of a box into which mail for a particular recipient is placed.

Noun

 * 1) A numerical address for postal correspondence, managed by a company or organisation on behalf of the addressee, and used e.g. for convenience, if one does not have a suitable physical address for receiving mail, or to avoid giving one's real address (e.g. in a newspaper advertisement).
 * They didn't give me a proper address, only a box number.
 * Would anyone with knowledge of the present whereabouts of Mrs Doris Miggins, formerly of Station Road, please contact Box No. 99, Puddletown Evening Gazette.

Usage notes
When forming part of an address, often capitalised as "Box No." or reduced to just "Box".