Germania (/dʒərˈmeɪniə/; Greek: Γερμανία Germanía) was the Roman and Greek term for the geographical region inhabited mainly by the Germanic people. It bordered to west on the Rhine river, to the south on the Danube river, to the north on the Baltic Sea, and to the east on the Vistula river. According to Friedrich Engels in his book The Origin of the Family, Private Property and the State (first published in 1884) Germania covered an area of 500,000 km or 190,000 sq mi and had a population of 5,000,000 in the 1st century BC. The areas west of the Rhine were mainly Celtic (specifically Gaulish) and became part of the Roman Empire in the first century BC.