What is Courier?

Legal Definition
One who is sent on some public occasion as an express, to bear despatches, letters, and other papers.

2. Couriers sent. by an ambassador or other public minister, are protected from arrest or molestation. Vattel, liv. 4, c. 9, 123.
-- Bouviers Law Dictionary
Legal Definition
A courier is a person who delivers messages, packages, and mail.
-- Wikipedia