Dated Precancel - a precancel stamp which includes the city as normal, but also includes the month and year of use. The "SRC" in the stamp at right stands for the company that used the precancel, the Sears Roebuck Company.

Dead Letter Office - a department of the post office that receives undeliverable mail without a return address. 

Definitive Stamp (also called "ordinary" or "regular" stamps) a postage stamp intended for routine postal use. Definitives are issued for an indefinite period of time and as many are printed as deemed necessary. A "commemorative" stamp on the other hand is issued for a definite period of time in pre-specified amounts.

Demonetized Stamp - a stamp that is no longer valid for postage. All stamps issued prior to the Civil War have been demonetized. Recently, Special Delivery stamps and Newspaper and Periodical stamps have been effectively demonetized, since the use for which they were intended is no longer valid. Note that airmail stamps are still valid for normal postage, even though they are no longer issued.

Denomination - the face value of a stamp, as printed on the stamp in either numerals or lettering.

Design - the image on a stamp

Designer - the artist who creates the artwork that is the basis for a stamp. Note that the designer does not actually engrave the stamp; that is the job of the "engraver".

Die - the piece of steel on which the design is engraved in reverse, or in mirror-image. After the artwork is approved, the die is the first step in the production of engraved stamps. The die is impressed many times on a printing plate to create multiple images of the stamp.

Die Flaw - any imperfection that existed on the surface of a blank die before the die was engraved. Often this imperfection appears on the printed stamps.

Die Proof - a printed impression made directly from the die at regular intervals as the die is being made, chiefly to aid the engraver in evaluating his progress and to make any corrections if necessary.

Discontinued Post Office - a post office no longer in operation.

Discount Postage - stamps sold below "face value". Most of the lower denomination U.S. stamps issued after 1940 can be bought in bulk often at as much as 20% or more discount.

Doane Cancel - Doane cancels were the result of the U.S. Post Office’s earliest attempts to improve postmark legibility by issuing rubber duplex handstamps. The name "Doane" is in honor of Edith Doane, a postal historian who became interested in these cancels and first published her results in 1978. Part of the handstamp contained the date, time, city and state of the post office and part contained a set of bars containing a numeral as a part of the cancel. The numeral indicated the annual compensation of the postmaster. The number "1" in the cancel meant that for the year prior to the issuing of the handstamp, the postmaster's annual compensation was less than $100. A numeral "2" meant that the compensation was between $100 and $200 and so on.

Docketing - a notation written by the recipient that records when a document was received and sometimes when it was answered, usually found on the outside of the document. Docketing is most often found on legal or commercial documents and often proves invaluable in establishing the date of a cover.

Documentary Stamps - (Scott’s "R") First used in 1862 to help pay for the Civil War, Documentary stamps are revenue stamps used to pay a federal tax on certain transactions that are "documented" on paper. The stamps were attached to the documents to show that the tax had been paid.

Domestic Mail - mail that never leaves the U.S. mail stream. International mail, on the other hand, leaves the U.S. mail stream and is handled by postal authorities from other nations.

Doremus Machine Cancel - Doremus machine markings started to appear in 1899, and continued until the 1930s in a few locations. Many are quite distinct.

Double Grill - a stamp that has been impressed by the grilling device twice.

Double Impression - a stamp with the design, or a portion of the design, doubled. A double impression is a freak caused primarily by slippage and should not be confused with a "double printing" or a "double transfer".

Double Joint Line - a joint line on rotary press stamps that appears to be two parallel lines, rather than the usual solid line. This phenomenon occurs when the gap between the two rotary plates is somewhat wider than normal. Note that a joint line is created when the ink spills over into the gap between the two rotary press plates attached to the printing cylinder. Joint lines are not to be confused with guide lines, which were printed intentionally, however similar they may appear.

Double Line Watermark - a watermark in which the initials USPS are outlined and hollow (double-line).
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Double Paper Stamp - a stamp printed on two layers of paper, sometimes intentional, as by the Continental Bank Note Co. in the printing of some of its Bank Note stamps. Double paper stamps were sometimes accidentally created when a tear or cut in the rotary press web was repaired with a splice.

Double Perforation - a stamp with an extra row or column of perforations. These stamps are considered to be "freaks", and not "errors"

Double Printed Stamp - a stamp that has passed through the printing press twice and shows two distinct overlapping images

Double Transfer - a plate variety in which a portion of the design is doubled. This occurs on engraved stamps when a design is "rocked" into a plate from a transfer roll in such a way that the resulting image is out of alignment. All sheets printed from a plate with a double transfer will show the double transfer in the same position and same stamp(s) on the sheet. Double transfers are collectible varieties.

Drop Letter - a letter mailed at a post office for delivery to another patron at the same office. Drop letters were charged less than letters that required dispatch to a different post office.

Dry Printing - the printing of engraved stamps on paper with a low moisture content and under high pressure, first used at the BEP in 1953. Before that stamps had been printed on moistened paper making it soft and pliable in order to press into the grooves of the engraved plates and transfer the ink under lower pressure.

Duck Stamps (Scott’s RW) - Officially known as "United States Migratory Bird Hunting and Conservation" stamps, "Duck" stamps have been issued annually since 1934. Other than the first Duck stamp of 1934, hunters were required to sign and affix the stamp to their hunting licenses. The most desirably duck stamps are autographed by the artist.

Duplex Canceling Machine - a handstamp uniting both a postmark and a cancel, introduced in the 1860s and in use through the 1940s.

Durland – The Durland Standard Plate Number Catalog is the authoritative catalog of plate numbers on U.S. stamps, originally published by Clarence Durland beginning in 1950, and now edited and published by the United States Stamp Society.