Crystal violet or gentian violet, also known as methyl violet 10B or hexamethyl pararosaniline chloride, is a triarylmethane dye used as a histological stain and in Gram's method of classifying bacteria. Crystal violet has antibacterial, antifungal, and anthelmintic (vermicide) properties and was formerly important as a topical antiseptic. The medical use of the dye has been largely superseded by more modern drugs, although it is still listed by the World Health Organization.
The name gentian violet was once used for a mixture of methyl pararosaniline dyes (methyl violet), but it is considered a synonym for crystal violet. The name refers to its colour, being like that of the petals of certain gentian flowers; it is not made from gentians or violets.