Taggant - a phosphorescent compound visible under ultraviolet light used to "tag" stamps for use by automated mail handling equipment

Territorial Cover - a cover postmarked in what was then a territory and not yet a state

Thin - literally, a thinned area on the back of a stamp, most often due to careless removal of a used stamp from its cover or from removing a hinge from the back of a stamp

Third Bureau Issue (The Washington-Franklins) - the third set of regular postage stamps issued by the Bureau of Engraving and Printing, sometimes known as the "Series of 1908"
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Third International Philatelic Exhibition (TIPEX) - the third in a series of once-per-decade international stamp shows hosted in New York City, this one in 1936. A souvenir sheet, Scott 778, was issued to promote the show.
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Tied - a stamp and cover "tied" by a cancellation or postmark. The stamp is considered "tied" when the cancellation covers at least part of the stamp and extends beyond the stamp onto the cover without break.

Toning (Foxing) - a discoloration of a stamp or cover from exposure to light, heat, or humidity

"TOP" - a marginal inscription placed on some bi-colored flat press stamps to reduce of the possibility of a sheet being inverted on its second pass through the press, and thus prevent the possibility of creating inverted stamps, as with the "Inverted Jenny" which in the case of the stamp at right also used the red, white and blue color scheme. Note that "TOP" is in red indicating the red frame of the design was printed first and that the blue vignette, printed second, shows some misregistration upwards and to the left.

Toppan, Carpenter, Casilear & Co. - the Bank Note engraving firm that held the contract to print U.S. postage stamps from 1851 until 1860
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Trans-Mississippi Issue - (Scott’s 285-293) a set of commemorative postage stamps issued to promote the Trans-Mississippi Exposition, held in Omaha in 1898
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Transportation Coils - an extensive series of definitive postage stamps, which began appearing in 1981. Many of these, such as the one at right, have plate numbers and some are highly collectible.

Transports - (Scott’s C25-C31) a series of air mail stamps issued in 1941, depicting a rendition of a transport-type aircraft
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Trial Color Proof - a proof made from a die or plate in a test color. This was the final step in the determination of the stamp’s ultimate color. Often trial proofs were made in many colors for this purpose.

Trimmed perforations - a side of a stamp that is normally perforated, but with the perforations cut away, often to imitate an imperforate or coil stamp of higher value

Tropical gum - gum that is heavily discolored due to fungal growth, caused by exposure to warm, humid conditions

Two Cent Reds - a nickname for the red 2¢ U.S. commemorative stamps issued between 1927 and 1932

Type - the term "type" is used on this web site to define a variation in a stamp design that is significant enough to warrant a separate Scott classification. The type will apply independently across variations in perforation, watermark, and printing method. 

Typographic Printing (see also relief printing) - a method of printing in which ink is transferred to the paper from a design that is raised from the surface of the plate, with the result that the ink appears to lay nearly flat or slightly below  the surface of the paper. Typographic printing is the opposite of intaglio or engraved printing in which the design is below the surface of the plate and the transferred ink lays above the surface of the paper (see also "The Art of Line Engraving")

Typography - the art of designing type, laying out type, setting type, or many other skills that are in the type field. Philatelists have incorporated the term "typography" into their lexicon, perhaps incorrectly, and given it a new meaning, one synonymous with "relief printing". The correct usage would be "typographic printing" (see above)