A Mongolian spot, also known as slate grey nevus or congenital dermal melanocytosis, is a benign, flat, congenital birthmark with wavy borders and an irregular shape. Wikipedia

Benign, flat, congenital birthmark, prevalent among East, South, Southeast, North and Central Asian peoples, Indigenous Oceanians, and Amerindians Wikidata

A benign, flat, congenital, usually blue to blue-black birthmark in lower back with wavy borders and irregular shape, prevalent among East Asians, South-East Asians, Polynesians, Native Americans, East Africans, and the Turkish people. It normally disappears three to five years after birth and almost always by puberty. Wiktionary

Congenital birthmark in lower back. Wiktionary (translation)