Obsolete : A stamp no longer sold by a post office though may still be valid for postage.

Official stamps : Stamps which are issued for use by government departments. e.g. Tax, Military.

On Paper : Stamps "on paper" are those that still have portions of the original envelope or wrapper stuck to them.

Optical Character Reader (OCR) : An automated mail processing machine that optically scans letter mail, locates the address and translates the address information into a bar code representation of the ZIP+4 delivery point bar code.

Overprint : Additional printing on a stamp added at a later stage, such as a change in postal rate or to denote a particular use and which was not part of the original design.