Devon (/ˈdɛvən/ DEV-ən; historically also known as Devonshire /-ʃɪər, -ʃər/ -⁠sheer, -⁠shər) is a ceremonial county in South West England. It is bordered by the Bristol Channel to the north, Somerset and Dorset to the east, the English Channel to the south, and Cornwall to the west. The city of Plymouth is the largest settlement, and the city of Exeter is the county town.

The county has an area of 2,590 sq mi (6,700 km²) and a population of 1,194,166. The largest settlements after Plymouth (264,695) are the city of Exeter (130,709) and the seaside resorts of Torquay and Paignton, which have a combined population of 115,410. They all are located along the south coast, which is the most populous part of the county; Barnstaple (31,275) and Tiverton (22,291) are the largest towns in the north and centre respectively. For local government purposes Devon comprises a non-metropolitan county, with eight districts, and the unitary authority areas of Plymouth and Torbay. Devon County Council and Torbay Council collaborate through a combined county authority.

Devon has a varied geography. It contains Dartmoor and part of Exmoor, two upland moors which are the source of most of the county's rivers, including the Taw, Dart, and Exe. The longest river in the county is the Tamar, which forms most of the border with Cornwall and rises in Devon's northwest hills. The southeast coast is part of the Jurassic Coast World Heritage Site, and characterised by tall cliffs which reveal the Triassic, Jurassic and Cretaceous geology of the region. The county gives its name to the Devonian geologic period, which includes the slates and sandstones of the north coast. Dartmoor and Exmoor have been designated national parks, and the county also contains, in whole or in part, five national landscapes.

In the Iron Age, Roman and the Sub-Roman periods, the county was the home of the Dumnonii Celtic Britons. The Anglo-Saxon settlement of Britain resulted in the partial assimilation of Dumnonia into the kingdom of Wessex in the eighth and ninth centuries, and the western boundary with Cornwall was set at the Tamar by king Æthelstan in 936.

The name Devon derives from the name of the Brythons 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 languages, 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.^{[citation needed]}

William Camden, in his 1607 edition of Britannia, described Devon as being one part of an older, wider country that once included Cornwall:

THAT region which, according to the Geographers, is the first of all Britaine, and, growing straiter still and narrower, shooteth out farthest into the West, [...] was in antient time inhabited by those Britans whom Solinus called Dumnonii, Ptolomee Damnonii [...] For their habitation all over this Countrey is somewhat low and in valleys, which manner of dwelling is called in the British tongue Dan-munith, in which sense also the Province next adjoyning in like respect is at this day named by the Britans Duffneit, that is to say, Low valleys. [...] But the Country of this nation is at this day divided into two parts, knowen by later names of Cornwall and Denshire, [...]

— William Camden, Britannia.