The Brittonic languages derive from the Common Brittonic language, spoken throughout Great Britain south of the Firth of Forth during the Iron Age and Roman period. In addition, North of the Forth, the Pictish language is considered to be related; it is possible it was a Brittonic language, but it may have been a sister language. In the 5th and 6th centuries emigrating Britons also took Brittonic speech to the continent, most significantly in Brittany and Britonia. During the next few centuries the language began to split into several dialects, eventually evolving into Welsh, Cornish, Breton, and Cumbric. Welsh and Breton continue to be spoken as native languages, while a revival in Cornish has led to an increase in speakers of that language. Cumbric is extinct, having been replaced by Goidelic and English speech. The Isle of Man and Orkney may also have originally spoken a Brittonic language, later replaced with a Goidelic one. Due to emigration, there are also communities of Brittonic language speakers in England, France, and Y Wladfa (the Welsh settlement in Patagonia).
The names "Brittonic" and "Brythonic" are scholarly conventions referring to the Celtic languages of Britain and to the ancestral language they originated from, designated Common Brittonic, in contrast to the Goidelic languages originating in Ireland. Both were created in the 19th century to avoid the ambiguity of earlier terms such as "British" and "Cymric". "Brythonic" was coined in 1879 by the Celticist John Rhys from the Welsh word Brython. "Brittonic", derived from "Briton" and also earlier spelled "Britonic" and "Britonnic", emerged later in the 19th century. It 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 that by that time "Brythonic" had become a dated term, and that "of late there has been an increasing tendency to use Brittonic instead." Today, "Brittonic" often replaces "Brythonic" in the literature. Rudolf Thurneysen used "Britannic" in his influential A Grammar of Old Irish, though this never became popular among subsequent scholars.
Comparable historical terms include the Medieval Latin lingua Britannica and sermo Britannicus and the Welsh Brythoneg. Some writers use "British" for the language and its descendants, though due to the risk of confusion, others avoid it or use it only in a restricted sense. Jackson, and later John T. Koch, use "British" only for the early phase of the Common Brittonic language.
Prior to Jackson's work, "Brittonic" (and "Brythonic") were often used for all the P-Celtic languages, including not just the varieties in Britain but those Continental Celtic languages that similarly experienced the evolution of the Proto-Celtic language element /kʷ/ to /p/. However, subsequent writers have tended to follow Jackson's scheme, rendering this use obsolete.
Knowledge of the Brittonic languages comes from a variety of sources. For the early languages information is obtained from coins, inscriptions and comments by classical writers as well as place names and personal names recorded by them. For later languages there is information from medieval writers and modern native speakers, together with place names. The names recorded in the Roman period are given in Rivet and Smith.