Basically, a person who contracts with a carrier for the carriage of goods by sea.
The legal definition, used in the IMSBC Code, is: “Any person by whom or in whose name, or on whose behalf, a contract of carriage of goods by sea has been concluded with a carrier, or any person by whom or in whose name, or on whose behalf the goods are actually delivered to the carrier in relation to the contract of carriage by sea.”