[4] "Brittonic" became more prominent through the 20th century, and was used in Kenneth H. Jackson's highly influential 1953 work on the topic, Language and History in Early Britain.

Jackson noted by that time that "Brythonic" had become a dated term: "of late there has been an increasing tendency to use Brittonic instead.

[5] Comparable historical terms include the Medieval Latin lingua Britannica and sermo Britannicus[6] and the Welsh Brythoneg.

[7][8] An early written reference to the British Isles may derive from the works of the Greek explorer Pytheas of Massalia; later Greek writers such as Diodorus of Sicily and Strabo who quote Pytheas' use of variants such as πρεττανική (Prettanikē), "The Britannic [land, island]", and νησοι βρεττανιαι (nēsoi brettaniai), "Britannic islands", with Pretani being a Celtic word that might mean 'painted ones' or 'tattooed folk', referring to body decoration.

[10] The Brittonic branch is also referred to as P-Celtic because linguistic reconstruction of the Brittonic reflex of the Proto-Indo-European phoneme *kʷ is p as opposed to Goidelic k. Such nomenclature usually implies acceptance of the P-Celtic and Q-Celtic hypothesis rather than the Insular Celtic hypothesis because the term includes certain Continental Celtic languages as well.

For all practical purposes Cornish died out during the 18th or 19th century, but a revival movement has more recently created small numbers of new speakers.

[14] A major archaeogenetics study uncovered a migration into southern Britain in the middle to late Bronze Age, during the 500-year period 1,300–800 BC.

During the period of the Roman occupation of what is now England and Wales (AD 43 to c. 410), Common Brittonic borrowed a large stock of Latin words, both for concepts unfamiliar in the pre-urban society of Celtic Britain such as urbanization and new tactics of warfare, as well as for rather more mundane words which displaced native terms (most notably, the word for 'fish' in all the Brittonic languages derives from the Latin piscis rather than the native *ēskos – which may survive, however, in the Welsh name of the River Usk, Wysg).

[18] It is probable that at the start of the Post-Roman period, Common Brittonic was differentiated into at least two major dialect groups – Southwestern and Western.

Between the end of the Roman occupation and the mid-6th century, the two dialects began to diverge into recognizably separate varieties, the Western into Cumbric and Welsh, and the Southwestern into Cornish and its closely related sister language Breton, which was carried to continental Armorica.

Substantial numbers of Britons certainly remained in the expanding area controlled by Anglo-Saxons, but over the fifth and sixth centuries they mostly adopted the Old English language and culture.

[19][20][21] The Brittonic languages spoken in what are now Scotland, the Isle of Man, and England began to be displaced in the 5th century through the settlement of Irish-speaking Gaels and Germanic peoples.

[18] The displacement of the languages of Brittonic descent was probably complete in all of Britain except Cornwall, Wales, and the English counties bordering these areas such as Devon, by the 11th century.

[24] The distribution of Proto-Celtic short vowels were reshuffled by various processes in Brittonic, such as the two i-affections, a-affection, raisings, and contact with lenited consonants like *g > /ɣ/ and *s > *h. The default outcomes of stressed short vowels in Brittonic are as follows: Welsh exhibits raisings of *e to *i > *ɪ > ɨ ⟨y⟩ and *o > /u/ ⟨w⟩ before a nasal followed by a stop.

Schrijver demonstrates this rule with the following examples:[48] Assuming that Welsh manach (borrowed from Latin monachus "monk") also underwent this assimilation, Schrijver concludes that this change must predate the raising of vowels in *mVn- sequences, which in turn predates a-affection (an early fifth-century process).

The sequence *ub > *uβ remained as such when followed by a consonant, for instance in Proto-Celtic *dubros "water" > *duβr > Welsh dwfr, dŵr and Breton dour.

[55] However, if no consonant exists after a *ub sequence, the *u merges with whatever Proto-Celtic *ou and *oi became, the result of which is written ⟨u⟩ in the Brittonic languages.

[61] The regular consonantal sound changes from Proto-Celtic to Welsh, Cornish, and Breton are summarised in the following table.

Names derived (sometimes indirectly) from Brittonic include London, Penicuik, Perth, Aberdeen, York, Dorchester, Dover, and Colchester.

The number of Celtic river names in England generally increases from east to west, a map showing these being given by Jackson.

These include Avon, Chew, Frome, Axe, Brue and Exe, but also river names containing the elements der-/dar-/dur- and -went e.g. Derwent, Darwen, Deer, Adur, Dour, Darent, and Went.

derow, W. derw), coupled with two agent suffixes, -ent and -iū; this is the origin of Derwent, Darent, and Darwen (attested in the Roman period as Deru̯entiō).

In Roman Britain, there were three tribal capitals named U̯entā (modern Winchester, Caerwent, and Caistor St Edmunds), whose meaning was 'place, town'.

It is generally accepted that Brittonic effects on English are lexically few, aside from toponyms, consisting of a small number of domestic and geographical words, which "may" include bin, brock, carr, comb, crag and tor.

[65][66][67] Another legacy may be the sheep-counting system yan tan tethera in the north, in the traditionally Celtic areas of England such as Cumbria.

Scottish Gaelic contains several P-Celtic loanwords, but, as there is a far greater overlap in terms of Celtic vocabulary than with English, it is not always possible to disentangle P- and Q-Celtic words.