Retinitis pigmentosa: A group of inherited disorders in which abnormalities of the photoreceptors (the rods and cones) of the retina lead to progressive visual loss. Abbreviated RP. People with RP first experience defective dark adaptation (night blindness), then constriction of the visual field (tunnel vision), and eventually, late in the course of the disease, loss of central vision. RP may be inherited in an autosomal dominant, autosomal recessive, or X-linked recessive manner or as a mitochondrial disorder. RP can occur alone or as part of a syndrome involving other abnormalities. More than 30 different genes are known to cause nonsyndromic RP (RP alone). Usher syndrome, which is RP and deafness, is a form of syndromic RP.