War stamp - a stamp issued during wartime to raise revenue for the war and inscribed or overprinted as such. The stamp was applied in addition to the regular postage.
Watermark - a pattern or design within the substance of the paper which is impressed in the wet paper pulp during the manufacturing process. Some stamps are printed on watermarked paper to prevent counterfeiting, identify the paper manufacturer, or identify different stamps or postal stationery.
Wiper marks - wavy smears found across whole sheets of stamps. Over-inking of the plate or a slow mechanical wiper can cause this result.