#	Term	Definition
51	Topical	Stamps that share a similar design and theme such as national parks, animals, space exploration, etc.  These are sometimes issued on the same page or souvenir sheets together.
52	UV Light	A black light that is used to view tagging that has been applied to certain stamps.  Lights with a wavelength 254 nm can be used for tagging on stamps from the United States, Canada, Israel, Great Britain, etc.  Lights with a wavelength of 366nm should be used on stamps of China, France, Mexico, etc.
53	Vignette	The area of a stamp on which the main design is located.
54	Watermark	Special marks that are incorporated into the stamp paper.  This makes it more difficult to counterfeit stamps.  Some watermarks may be seen with the naked eye while some require a special solution to view them.