No fixed abode

In law, no fixed abode or without fixed abode is not having a fixed geographical location as a residence. This is applicable to several groups:

A person with no fixed address may have mail addressed to a Poste restante service. Such persons may also have problems receiving government social services and registering to vote, and may be deprived of many common services.

Ships may have a mobile letter box, that can be placed on the dockside whilst a ship is moored in port or the crew are on shore leave, allowing the delivery of mail to those living on the ship—for as long as the vessel is in that port.

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Wikipedia