Devon derives its name from Dumnonia, which, during the British Iron Age, Roman Britain, and Early Medieval was the homeland of the Dumnonii Brittonic Celts. The Anglo-Saxon settlement of Britain resulted in the partial assimilation of Dumnonia into the Kingdom of Wessex during the eighth and ninth centuries. The western boundary with Cornwall was set at the River Tamar by King Æthelstan in 936. Devon was constituted as a shire of the Kingdom of England thereafter.
The north and south coasts of Devon each have both cliffs and sandy shores, and the county's bays contain seaside resorts, fishing towns, and ports. The inland terrain is rural, generally hilly, and has a low population density in comparison to many other parts of England. Dartmoor is the largest open space in southern England at 954 km2 (368 square miles), its moorland extending across a large expanse of granite bedrock. To the north of Dartmoor are the Culm Measures and Exmoor. In the valleys and lowlands of south and east Devon the soil is more fertile, drained by rivers including the Exe, the Culm, the Teign, the Dart, and the Otter.
As well as agriculture, much of the economy of Devon is linked with tourism. The comparatively mild climate, coastline and landscape give rise to Devon as a destination for recreation and leisure in England, with visitors particularly attracted to the Dartmoor and Exmoor national parks; its coasts, including the resort towns along the south coast known collectively as the English Riviera, the Jurassic Coast, and North Devon's UNESCO Biosphere Reserve; and the countryside including the Cornwall and West Devon Mining Landscape.
The name Devon derives from the name of the Britons who inhabited the southwestern peninsula of Britain at the time of the Roman conquest of Britain known as the Dumnonii, thought to mean "deep valley dwellers" from proto Celtic *dubnos 'deep'. In the Brittonic, Devon is known as Welsh: Dyfnaint, Breton: Devnent and Cornish: Dewnens, each meaning "deep valleys." (For an account of Celtic Dumnonia, see the separate article.) Among the most common Devon placenames is -combe which derives from Brittonic cwm meaning 'valley' usually prefixed by the name of the possessor.
William Camden, in his 1607 edition of Britannia, described Devon as being one part of an older, wider country that once included Cornwall: