Cornwall ( /ˈkɔrnwɔːl/ or /ˈkɔrnwəl/;Cornish: Kernow [ˈkɛrnɔʊ]; Latin: Cornubia or Cornuvia) is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of 537,400 and covers an area of 3,563 km² (1,376 sq mi). The administrative centre and only city is Truro.

The area now known as Cornwall was first inhabited in the Palaeolithic and Mesolithic periods. It continued to be occupied by Neolithic and then Bronze Age peoples, and later (in the Iron Age) by Brythons with distinctive cultural relations to neighbouring Wales and Brittany. There is little evidence that Roman rule was effective west of Exeter and few Roman remains have been found. Cornwall was a division of the Dumnonii tribe—whose tribal centre was in the modern county of Devon—known as the Cornovii, separated from Wales after the Battle of Deorham, often coming into conflict with the expanding English kingdom of Wessex before King Athelstan in AD 936 set the boundary between English and Cornish at the Tamar. From the early Middle Ages, British language and culture was apparently shared by Brythons trading across both sides of the Channel, evidenced by the corresponding high medieval Breton kingdoms of Domnonee and Cornouaille and the Celtic Christianity common to both territories.