Imperforate: Refers to stamps without perforations or rouletting between the individual stamps in a pane. The earliest stamps were imperforate by design, but after about 1860 most stamps were perforated. Modern imperforates are usually errors or are produced specifically for sale to stamp collectors.
Impression: Any stamped or embossed printing.
Imprimatur: Latin for “let it be printed.” The first sheets of stamps from an approved plate, normally checked and retained in a file prior to a final directive to begin stamp production from a plate.
India paper: A thin, tough opaque printing paper of high quality used primarily for striking die proofs.
Indicium: The stamp impression of a postage meter or the imprint on postal stationery (as opposed to an adhesive stamp), indicating prepayment and postal validity.
Information sheets: This is an official document issued by the department of Post with every stamp issue. It gives brief history of the subject of stamp, technical data and description of the stamp design as well as of the first day cover and first day postmark.
Inscription: The letters, words and numbers that are part of a postage stamp design.
Instanta: A modern perforation gauge made by Stanley Gibbons.
Intaglio: Italian for “in recess.” A form of printing in which the inked image is produced by that portion of the plate sunk below the surface. Line engraving and gravure are forms of intaglio printing.
Invert: The term generally used to describe any error where one portion of the design is inverted in relation to the other portion(s). An overprint applied upside down is also an invert.
IRC: International Reply Coupon: A redeemable certificate issued by member nations of the Universal Postal Union to provide for return postage from recipients in other countries. IRCs are exchangeable for postage at a post office.