Term used for the ethnic groups speaking related languages in eastern Europe during the second half of the 1st millennium AD. They inhabited an area concentrated in modern Poland, and by the early Middle Ages they were considered a distinct cultural group. The origins of the Slavs are obscure, though they seem to derive from the Iron Age tribes indigenous to the Oder-Vistula area. Prehistorically, the original habitat of the Slavs was Asia, from which they migrated in the 3rd or 2nd millennium BC to populate parts of eastern Europe. Subsequently, these European lands of the Slavs were crossed or settled by many peoples forced by economic conditions to migrate. State-level polities began in Greater Moravia in the 9th century AD and in Poland in the 10th century. They are principally defined by linguistic and place-name evidence rather than by historical or archaeological remains. The gród or hrad ("castle") was the stronghold of Slav communities. It is the most numerous ethnic and linguistic body of peoples in Europe residing also across northern Asia to the Pacific Ocean. Slavic languages belong to the Indo-European family. Customarily Slavs are subdivided into east Slavs (Russians Ukrainians and Belarusians) west Slavs (Poles Czechs Slovaks and Wends or Sorbs) and south Slavs (Serbs Croats Slovenes and Macedonians).