An Ultraviolet marker is a pen whose marks are fluorescent

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 but transparent: the marks can be seen only under an ultraviolet light. They are commonly used in security situations to identify belongings or to protect from the copying of banknotes. UV pens can now be bought at some stationery

Stationery

Stationery has historically meant a wide gamut of materials: paper and office supplies, writing implements, greeting cards, glue, pencil case etc.-History of stationery:...

shops to security mark items of high value in case of theft.