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:
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.
The term "Devon" is normally used for everyday purposes e.g. "Devon County Council" but "Devonshire" continues to be used in the names of the "Devonshire and Dorset Regiment" and "The Devonshire Association". One erroneous theory is that the "shire" suffix is due to a mistake in the making of the original letters patent for the Duke of Devonshire, resident in Derbyshire. However, there are references to "Defenascire" in Anglo-Saxon texts from before 1000 AD (this would mean "Shire of the Devonians"), which translates to modern English as "Devonshire". The term Devonshire may have originated around the 8th century, when it changed from Dumnonia (Latin) to Defenascir..
Kents Cavern in Torquay had produced human remains from 30–40,000 years ago. Dartmoor is thought to have been occupied by Mesolithic hunter-gatherer peoples from about 6000 BC. The Romans held the area under military occupation for around 350 years. Later, the area began to experience Saxon incursions from the east around 600 AD, firstly as small bands of settlers along the coasts of Lyme Bay and southern estuaries and later as more organised bands pushing in from the east. Devon became a frontier between Brittonic and Anglo-Saxon Wessex, and it was largely absorbed into Wessex by the mid 9th century. A genetic study carried out by the University of Oxford & University College London discovered separate genetic groups in Cornwall and Devon, not only were there differences on either side of the Tamar, with a division almost exactly along the modern county boundary dating back to the 6th Century but also between Devon and the rest of Southern England, and similarities with the modern northern France, including Brittany. This suggests the Anglo-Saxon migration into Devon was limited rather than a mass movement of people.
The border with Cornwall was set by King Æthelstan on the east bank of the River Tamar in 936 AD. Danish raids also occurred sporadically along many coastal parts of Devon between around 800AD and just before the time of the Norman conquest, including the silver mint at Hlidaforda Lydford in 997 and Taintona (a settlement on the Teign estuary) in 1001.
Devon has also featured in most of the civil conflicts in England since the Norman conquest, including the Wars of the Roses, Perkin Warbeck's rising in 1497, the Prayer Book Rebellion of 1549, and the English Civil War. The arrival of William of Orange to launch the Glorious Revolution of 1688 took place at Brixham.
Devon has produced tin, copper and other metals from ancient times. Devon's tin miners enjoyed a substantial degree of independence through Devon's Stannary Parliament, which dates back to the 12th century. The last recorded sitting was in 1748.