Railway Post Office (RPO) and Railway Postmarks - a portable post office that sorts and processes mail in transit

Rawdon, Wright, Hatch & Edson - the New York security firm that in 1847 printed the first U.S. Postage Stamps, Scott’s 1 and 2. The firm’s initials "RWHE" may be found in the bottom margin of each stamp.
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Receiving Mark - a postmark usually applied to the back cover of the mail indicating the name of the receiving post office, the date and sometimes the time of arrival

Recut - a printing plate that has been retouched

Redrawn - a new stamp design that maintains most of the basic design of the originally issued stamp

Re-engraved - a plate or die on which a portion of the original design is altered. This is sometimes done to strengthen the worn areas on the plate. Stamps from such re-engraving create new and often collectible varieties.

Re-entry - a re-entry is a plate repair to fix a defective entry or to extend the life of a worn plate by re-rocking the transfer roll over the defective impression. The re-entry is detectable only when it is not perfectly aligned with the existing impression.

Registered Mail - the most secure way to send mail, with each postal employee that handles it signing a receipt

Registration Stamp - (Scott F1) In 1911 a stamp was issued to pay the registration fee of ten cents. 
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Regummed - a stamp that has had gum added in an attempt to fool collectors into thinking the stamp has original gum. Since the gum can easily be removed, a regummed stamp is considered as an NG (no gum) stamp for valuation purposes. However, the presence of undocumented regummed stamps will lower the value of a collection, all things considered, since it is considered either an outright attempt to deceive or shows a lack of diligence on the part of the collector. 

Regular Issue - (a.k.a. "definitive" or "ordinary issue") a stamp issued for an unspecified period and in non-predetermined quantities. Note that commemoratives have a limited lifespan and are issued in limited quantities.

Re-issue - an official re-printing of a stamp that is no longer issued by the Post Office

Relief Printing (Letterpress or Embossing) - the opposite of engraved, the stamp’s design is raised from the surface of the transfer roll, resulting in the stamp’s image being pressed into the paper

Relief Break - a relief break is not a break on the engraved sheet, it is the breaking away of part of the transfer roll that places the stamp’s image on the engraved plate. Thus there will be no ink applied in the area of the relief break.

Remainders - a collection of stamps in which the most valuable or most desirable items have been removed

Reperforated - a stamp that has had perforations added to one or more sides to defraud a collector. At one time it was a common practice, and not considered entirely unethical (as it is today), to add perforations to the natural straightedge side of a stamp to make it more pleasing in appearance.

Reprint - a later printing of a stamp from the original plate, generally to meet public demand, but sometimes for presentation purposes. Often, Reprints can be distinguished from the originals.

Reproductions - stamps made from a new plate designed to imitate the original issue. Scott’s 3 and 4 are reproductions of Scott’s 1 and 2. Reproductions can nearly always be distinguished from the originals.

Retouch - a repair to a flaw in a die or plate. The retouching often results in a collectible variety.

Revenue Stamp - a stamp issued to pay various types of taxes, duties and fees other than for postage. 

Reversed Watermark - If the watermarked paper is placed in the press so that the front of the paper is facing down, the watermark will appear revered as viewed from the front. 

RFD - an acronym for "Rural Free Delivery"

Ribbed Paper - a paper that clearly shows fine parallel lines running through it. Ribbed paper is found on many of the Bank Notes. In fact, the presence of ribbed paper on a single copy of the twenty-four cent Bank Note, is the source of one of the great controversies in U.S. philately, Scott 164, the twenty-four cent Continental. 

Rocking-in - the part of the engraved printing process when a transfer roll is "rocked" over the engraved plate.

Rodgers Aerial Post (Vin Fiz Flyer) - a semi-official stamp issued to carry mail on a 1911 cross-country airplane flight.

Roosevelt Presentation Albums - In 1903, during the presidency of Theodore Roosevelt, the Post Office made an estimated 85 Presentation Albums of sets of the die proofs of all U. S. postage stamps designs to date, as gifts to various dignitaries.

Rosback Perforation - (Found on Scott 536) an experimental 12.5 gauge perforation used by the Bureau of Engraving and Printing on the one cent offset stamp of 1919. We have seen this spelled "Rossback", "Rossbach" and "Rosbach" as well. The Rosback company is still in business and has a web presence. They are now "a world-leading designer and manufacturer of quality handcrafted book binding and print finishing equipment for the graphic arts industry". more...

Rosette Crack - fine cracks radiating from a central point on an engraved plate.

Rotary Press Stamps - stamps printed from curved plates fastened around a cylinder on a continuous roll of paper as opposed to the flat plate press which printed only one sheet of paper at a time. Rotary press stamps are slightly stretched, that is longer or wider, as the roll of paper is pulled through the press.

Rotogravure (Rotary Photogravure) - the process of making an engraved plate by photographic means to be used to print on a rotary press

Roulette - a perforation that involves making short dash-like slits in the paper between stamps without actually removing any paper. Rouletted perforations are commonly found on Revenue stamps.

RPO - an acronym for "Railway Post Office"

Rural Free Delivery (RFD) - a service of the Post Office, begun as an experiment in rural West Virginia on October 1, 1896, that provided free mail delivery to homes, primarily farmers, outside of the urban areas

Rust - similar in appearance to "foxing", rust is a reddish brown mold resembling the rust in iron, disfiguring stamps in tropical, humid climates

R.W.H.E. - the initials for Rawdon, Wright, Hatch & Edson, the manufacturers of the first U.S. stamps in 1847. The letters appear in the bottom margins of Scott’s 1 and 2 and the reprints of 1875, Scott’s 3 and 4.