Definitive issue - a stamp issued in an indefinite quantity and for an indefinite period, usually several years or more. The United States presidential issue of 1938 and the 1995 32-cent Flag over Porch stamps are examples. Definitive stamp designs do not honor a specific time-dated event.
Die - the original engraving of a stamp design, usually recess-engraved in reverse on a small, flat sheet of soft steel. In traditional intaglio printing, a transfer roll is made from a die, and printing plates are made from impressions on the transfer roll. When more than one die is used in the production of an issue, distinctive variations are often identifiable.
Double entry - a term which refers to a visual characteristic on an engraved stamp such as partial doubling of the design due to misalignment of the transfer roll or over-rocking of a transfer roll which carries more than one design