Lichen planus (LP) is a disease characterized by itchy reddish-purple polygon-shaped skin lesions on the lower back, wrists, and ankles. It may also present with a burning sensation in the mouth, and a lattice-like network of white lines near sites of erosion (Wickham striae). The cause is unknown, but it is thought to be the result of an autoimmune process with an unknown initial trigger. There is no cure, but many different medications and procedures have been used in efforts to control the symptoms.

The term lichenoid reaction (or lichenoid lesion) refers to a lesion of similar or identical histopathologic and clinical appearance to lichen planus (i.e., an area which resembles lichen planus, both to the naked eye and under a microscope