but the proper term is set-off.
An age old argument. There really is no proper term. Me, I'm an offset guy.
This glossary defines nearly 300 terms frequently encountered by stamp collectors and cover collectors. Precise definitions for many philatelic terms do not exist. One collector, dealer or society may define a term in one way, while others will use the term in a slightly different way.
For special uses of some of the terms listed and defined here, contact the appropriate specialist collector group.
Offset: 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.