Viologens are organic compounds with the formula (CHNR).  In some viologens, the pyridyl groups are further modified.
Viologens are called so, because these compounds produce violet color on reduction [violet + Latin gen, generator of]. 
The viologen paraquat (R = methyl), is a widely used herbicide. As early as in the 1930s, paraquat was being used as an oxidation-reduction indicator, because it becomes violet on reduction.
Other viologens have been commercialized because they can change color reversibly many times through reduction and oxidation.  The name viologen alludes to violet, one color it can exhibit, and the radical cation (CHNR) is colored intensely blue.