What is "mongolian spot"

A Mongolian spot, also known as Mongolian blue spot, congenital dermal melanocytosis, and dermal melanocytosis is a benign, flat, congenital birthmark with wavy borders and irregular shape. In 1883 it was described and named after Mongolians by Erwin Bälz, a German anthropologist based in Japan. It normally disappears three to five years after birth and almost always by puberty. The most common color is blue, although they can be blue-gray, blue-black or deep brown.