I

IBI: Information-Based Indicia, printed designs indicating prepayment of postage.
I C: Iron Curtain
IDEM: This word, when printed on top of a stamp issue indicates that it has all the indications of the previous issue.
IFSDA: International Federation of Stamp Dealers Association.
Illegal Use: An improper use of a postage stamp or other adhesive; may be intentional or unintentional.
ILLUSTRATED COVERS: A cachet with an illustration, not just words.
IMP: Imperforate
IMPERFORATE (IMPERF.): Stamps without separating holes. They are usually separated by scissors and are collected in pairs.
IMPERFORATE BETWEEN: A pair of stamps with perforations on all four sides with either the horizontal or vertical perforations completely omitted.
IMPRESSION: A printed sheet is an "impression" taken from the printing press or plate. Also known as the actual printed design of a stamp.
IMPRIMATUR: Term for the first pane of a stamp produced after a printing plate was approved.
IMPRINT BLOCK: A block of stamps taken from a portion of the sheet where the printer's name or imprint is located on the margin.
IMPRINTS: Stamps issued from 1851 until 1917 bore imprints in the margin of the panes to identify the manufacturer. The Bureau of Engraving and Printing stopped putting imprints on panes in 1911, but they have appeared in later years when old plates with imprints were used to produce additional stamps.
IMPROVED PERFORATIONS: Where several perforations have been changed to enhance the appearance, such as punching out of unpunched perforations.
INCLUSIONS: Any substances incorporated in the paper web during the manufacturing process and normally different in color from the stamp.
INDEPENDENT MAILS: Express companies in 1844-45 that initiated mail service along railroad and water routes between cities in the northeast and also to and from the Great Lakes region.
INDIA PAPER: A strong paper that is soft, thin and silk-like usually used for proof impressions.
INDICIA: Stamped impression of the denomination indicating prepayment of postage.
INFLATION ISSUE: Stamps issued during high periods of financial instability.
INFORMATION-BASED INDICIA: A postage imprint that features a two-dimensional bar code containing data necessary for revenue protection.
INITIALS: Used in sheet margins as identification of individuals working at the Bureau of Engraving and Printing who participated in the production of plates.
INLAND MAIL STAMPS: Stamps intended specifically for domestic use.
INSCRIPTION: Any lettering or numbers on a postage stamp.
INSELPOST: German stamps used to carry mail to and from Crete, Rhodes and Aegean islands occupied by Germany during WWII.
INSURANCE STAMPS: Stamps issued for use by a government's life insurance department.
INSURED LETTER STAMPS: Stamps issued to pay the insurance fee on a letter.
INTAGLIO: Any type of printing in which the inked image is below the surface of the plate.
INTELPOST: Electronic facsimile system for transmission of documents used by postal administrations.
INTERIOR SERVICE: Stamps not valid for UPU service but may be the first stamps of several nations such as Cape of Good Hope, Reunion, Switzerland. Also all the stamps of some other nations such as Shanghai and the small Indian states.
INTERLEAVES: Translucent tissue placed between the pages of an album to prevent stamps from rubbing against each other.
INTERNAL REVENUE: The revenue tax collected within the country.
INTERNEE MAIL: Mail from persons interned during time of war.
INTRA-CITY POSTS: Served the local community by taking mail to and from the post office and delivering letters between correspondents in the same area; same as local posts.
INVALIDATED: No longer valid for postal use.
INVERT: Usually a multicolored stamp in which one of the colors or the design has been printed upside down. For example, the 24 cents air mail issue of 1918 in which the airplane, printed in blue, is upside down.
INVERTED CENTER: Stamp in which the central design is upside down in relation to the frame of the stamp.
INVERTED JENNY: U.S. 1918 24-cent air mail error.
INVERTED SWAN: Australia stamp with frame inverted issued in 1855.
INVERTED WATERMARK: Since the watermark is the first marking the paper receives, the watermark can not be inverted, only the printing can be inverted or sideways.
INVISIBLE GUM: Colorless and tasteless gum on the reverse of a stamp.
I P S A: Independent Postal System of America, founded on February 14, 1968, as an alternate to the USPS.
I R C: International Reply Coupon is the Universal Postal Union method to provide postage for correspondents in other countries.
IRREGULAR PERFORATIONS: Perforations where the holes are not aligned, are different sizes, are misplaced or are in any manner, abnormal.
I S J P: International Society for Japanese Philately
ISSUE: Act of a new stamp, or series of stamps, being released by a postal authority.
ISSUED (NON): A catalog term for a prepared stamp that has not been issued for various reasons.
I W Y: Inernational Women's Year