A benign, flat, congenital birthmark, with wavy borders and an irregular shape. The color is caused by melanocytes, melanin-containing cells, that are usually located in the surface of the skin (the epidermis), but are in the deeper region (the dermis) in the location of the spot. The disease is mongolian spot (MONDO_0006582, a Monarch Disease Ontology entry). Also known as: Mongolian macula, blue sacral spot, congenital dermal melanocytosis.