Illegal Use - an improper use of a stamp in an attempt to pay postage either intentionally or unintentionally

Imperforate - a stamp that has no perforations. Until 1857, U.S. stamps were issued imperforate. Most U.S. postage stamps were given perforations since then, but a few were issued imperforate in order that private companies could make coil stamps that worked with their affixing machines.

Imperforate Between - occasionally a sheet of stamps is issued with a row or column missing perforations. Adjoining stamps missing these perforations are called "imperforate between" and usually command a premium. Care must be taken not to confuse blind perforations with no perforations, if there is even the slightest suspicion of a perf dimple, the stamp must be considered to have blind perfs and not "imperforate between". 

Imprimatur - an early "test" printing of a sheet of stamps from a new printing plate. The sheet need not be made on the same paper as the issued stamps, nor in the same color, and the sheet often remains ungummed and imperforate.

Imprint - the name or initials of the printer or any other information printed in the margin of a sheet of stamps. The plate number is not considered an imprint.

Imprint Block - a block of stamps removed from the sheet in such a way that the imprint, usually the name of the printer, remains intact. 

Improved Perforations - perforations that are altered to improve the appearance of the stamp. This would include punching out a blind perforation. These are not ethical alterations and will actually lower the value of the stamp if detected.

Inclusion (Natural Inclusion) - any imperfection embedded in the stamp often of significantly different color than the stamp itself. Inclusions usually detract from the value of the stamp, but often only marginally.

India Paper - a very thin, soft, silky textured, hand-made, opaque paper made from bamboo, primarily used for making die proofs. Since India paper was hand-made, the proofs often show irregular spots resembling thins or even pin holes.

Intaglio - Any type of printing in which the design is recessed, that is below the surface of the plate. The design is etched into the surface of the plate by engraving, a mechanical method, or by gravure, a chemical method.

Interleaves - Sheets of glassine paper used to separate sheets of stamps, stamp album pages, and stamp booklet panes, among other things.

International Vending Machine Company Perforations - private perforations applied to stamps by the International Vending Machine Company for use in its vending machines. These were the first government authorized experimental vending machines, first used in Washington, D.C. in 1908.
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Invert - An invert is an error stamp in which a part of the design is printed "upside down" in relation to the rest of the design. Note that an invert can only occur when the printing process requires at least two independent passes through the press. Most inverts have at least two colors in which each color was printed separately, allowing a careless operator the chance to place the sheet in the press upside down on the second run.

Inverted Jenny - The nickname for the 24¢ inverted airmail stamp of 1918 on which the airplane (vignette) was printed upside down in relation to the frame.

Inverted Watermark - An inverted watermark is upside down in relation to the design on the stamp. 

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