Perioikos is the singular form of perioikoi. The periokoi or "dwellers around," were free men of Sparta, mainly farmers and merchants who lacked the full citizenship of the Spartiates. They lived in perhaps 80 or 100 towns and villages, which were called poleis
(the plural of polis),
in the less fertile land of the hills and coasts. They may have been part of the conquered people, but unlike the helots, they kept their freedom. Perhaps they were conquered earlier; perhaps the helot serfs were periokoi who rebelled.
See The Spartans - The World of the Warrior - Heroes of Ancient Greece, by Paul Cartledge