Xeroderma pigmentosum: An inherited skin disorder characterized by photosensitivity with severe sunburn in infancy, the development of numerous pigmented spots resembling freckles, larger atrophic lesions associated with telangiectasis, and multiple solar keratoses.eroderma pigmentosum is listed as a  rare disease  by the Office of Rare Diseases (ORD) of the National Institutes of Health (NIH). This means that Xeroderma pigmentosum, or a subtype of Xeroderma pigmentosum, affects less than 200,000 people in the US population.