Postage stamps provided for use on official correspondence. Where they are inscribed or overprinted for the use of specific departments they are known as Departmental stamps. The famous VR Penny Black of 1840, prepared for use but never issued, was intended for this purpose. Official stamps may be recognised by their inscription or overprint, e.g. "Service", "Dienstmarke", "Tjaneste", "Offentlig Sak" or variants of "Official" itself.

1) A printing process that transfers an inked image from a plate to a roller. The roller then applies the ink to paper. 2) The transfer of part of a stamp design or an overprint from one sheet to the back of another, before the ink has dried (also called set off). Such impressions are in reverse (see Mirror image). They are different from stamps printed on both sides.

The adhesive coating on a mint or unused stamp or envelope flap applied by a postal authority or security printer, usually before the item was issued. Upon request of stamp collectors, postal authorities have at times offered to add gum to items first issued ungummed. See also Regummed.

Abbreviation for On His (or Her) Majesty's Service. Used in perfins, overprints or franks to indicate Official use in the British Commonwealth.

An issue of stamps made by a number of countries simultaneously to commemorate the same person or event, and formerly using uniform designs. The first omnibus issue was made in 1898 by Portugal and her overseas colonies for the quatercentenary of Vasco da Gams's discovery of the sea route to India. Subsequently France (1931) and the British crown colonies (1935) produced omnibus issues. The latter continued to use uniform designs till 1967, but since then greater variety has been imparted by using distinctive motifs, although they tend to have uniform frames.