Generally, packet trade is any regularly scheduled cargo, passenger and mail trade conducted by ship. The ships are called "packet boats" as their original function was to carry mail.

A "packet ship" was originally a vessel employed to carry post office mail packets to and from British embassies, colonies and outposts. In sea transport, a packet service is a regular, scheduled service, carrying freight and passengers. The ships used for this service are called packet ships or packet boats. The seamen are called packetmen, and the business is called packet trade.

"Packet" can mean a small parcel but, originally meant a parcel of important correspondence or valuable items, for urgent delivery. The French-language term "paquebot” derives from the English term "packet boat," but means a large ocean liner.

This sense became extended to mean any regularly scheduled ship, carrying passengers, as in packet trade. The word "packet" is frequently modified by the destination, e.g. Sydney packet, or by motive force, e.g. "steam packet".