Underprint: A fine printing underlying the design of a stamp, most often used to deter counterfeiting.
Ungummed: A stamp without gum. Ungummed stamps are either stamps issued without gum or an uncancelled gummed stamp that has had its gum soaked off. Many countries in tropical climates have issued stamps without gum.
Unhinged: A stamp without hinge marks, but not necessarily with original gum.
Unissued stamps: Stamps printed but were not officially released for sale and used by the postal authorities.
Universal Postal Union – (UPU): An international organization formed in Bern, Switzerland, in 1874, for arranging the interchange of mail, standardize postal usage and to facilitate the movement of mail between member nations and most nations belong to the Universal Postal Union.
Unknown cancellations: A cancellation to denote that the addressee could not be traced. It may be due to want of correct address or the addressee left the place without leaving onward address to anybody. Catches like ‘Unknown’ ‘Return to sender’ or ‘Refused’ were commonly used in times.
Unmounted Mint: Stamps in pristine condition, as issued from post office, and which have never been hinged or mounted in any way. Such stamps tend to command a premium over mounted mint or used stamps.
Unused: Stamps, without post office seals but with gum having been wiped off due to some reason, are called unused.
Used abroad: Stamps of one country used and postmarked in another country.
Used: The condition of a stamp when it has been cancelled, by a postal authority to prevent its reuse on mail. In general, stamps, with post office seals, are called used.