Illustrated envelopes - Illustrated or imprinted envelopes often used for advertising and propaganda.

Imitation - A stamp produced by the postal administration deliberately imitating a design when the original plates are no longer available for reprints. An imitation might differ from the originals in paper, gum, size, color, and design detail. Imitations are produced to fill gaps in official collections and exhibitions, as well as for sale to collectors.

Imprimé - (French) Printed matter.

Imprint - Any detail of a stamp's design printed at the time of the original item production; inscription with name, initials, etc., usually of the producers of the stamps. The latter is found in the margins of sheets of the stamps.

Intaglio - From the Italian, to engrave, is a process by which a metal plate, traditionally copper, is incised by tools or acid baths to create a reliefed surface. Ink is pushed into these depressions and after the surface is wiped clean, paper is then pressed into these depressions under great pressure from a metal rolling (cylinder) press transferring the image.

International reply coupon - Receipts issued by member nations of the Universal Postage Union for return-postage payment. The certificates may be exchanged in foreign countries for local stamps.

Interrupted mail - Mail which has been detained or delayed.

Imperforate - The lack of some separation device between stamps, such as perforation holes or rouletted slits that allow for easy separation. Imperforate stamps have to be cut apart by scissors or blades. Most early issues were imperforate, and had to be hand-cut by the postmaster.

IDF - Israel Defense Force