Railway Post Office - train cars that were the property of railway companies but used by and restricted to U.S. Post Office Department employees. The RPO cars were usually placed directly behind the locomotive.
Regional stamp - a stamp issued by Great Britain (beginning in 1958) and sold in Scotland, Wales, Northern Ireland, Guernsey, Jersey, and Isle of Man but valid for postage throughout the United Kingdom
Registered Mail - mail that is recorded by the post office when sent. It is also recorded along points in the route to track it. Because registered mail has often been used for valuable properties, it was often a prime target in mail robberies.
Relief printing - a term used to designate printing from a base in which those areas to appear in color on the stamps are raised above the non-printing areas. Only the color-producing areas come into contact with the paper at the moment of impression or printing. Definition taken from "Fundamentals of Philately" by L. N. Williams.
Remainders - unsold stamps of a discontinued issue that are normally destroyed but can be overprinted, surcharged, or revalidated for postage. They are sometimes sold to collectors in their original state, cancelled, or overprinted to show they have been withdrawn from use.
Rotary press - a printing press that utilizes curved plates that rotate rapidly, printing on a continuous roll of paper. Stamps printed on a rotary press are usually longer or wider than the same stamps printed from flat plates because rotary plates stretch during the curving process
Rural Free Delivery (RFD) - mail service that provides delivery to rural customers. The service was established in 1896. Prior to RFD, farm families traveled miles to the post office to pick-up their mail, all the while paying the same rates as those living in towns and urban areas. During the service's early years, the carrier postmarked the items. For decades Congress had been reluctant to institute free rural delivery, seeing the nation as too large for such a service and predicting financial disaster. RFD became an official part of the U.S. Postal Service in 1902.