The Britons (sometimes Brythons or British) were the Celtic people culturally dominating Great Britain from the Iron Age until the Early Middle Ages. They spoke the Insular Celtic language known as British or Brythonic. They lived throughout Britain south of about the Firth of Forth; after the 5th century Britons also migrated to continental Europe, where they established the settlements of Brittany in France and the obscure Britonia in what is now Galicia, Spain. Their relationship to the Picts north of the Forth has been the subject of much discussion, though most scholars accept that the Pictish language during this time was a Brythonic language related to, but perhaps distinct from, British.

The earliest evidence for the Britons and their language in historical sources dates to the Iron Age. After the Roman conquest of 43 AD, a Romano-British culture began to emerge. With the advent of the Anglo-Saxon settlement in the 5th century, however, the culture and language of the Britons began to fragment. By the 11th century their descendants had split into distinct groups, and are generally discussed separately as the Welsh, Cornish, Bretons, and the people of the Hen Ogledd ("Old North"). The British language developed into the distinct branches of Welsh, Cornish, Breton, and Cumbric.

Black British is a term used to describe British people of black African descent, especially those of Afro-Caribbean background. The term has been used from the 1950s to refer to Black people from former British colonies in the West Indies (i.e. the New Commonwealth) and Africa, who are residents of the United Kingdom and consider themselves British. Others are also from former French-speaking colonies in Africa such as Senegal and the Democratic Republic of the Congo (which was Belgian), and many of the Black Africans in Britain still speak French as well as their own native languages.

The term 'black' has historically had a number of applications as a racial and political label, and may be used in a wider socio-political context, to encompass a broader range of non-European ethnic minority populations in Britain, though this is a controversial and non-standard definition.

Black British is primarily used as an official category in UK national statistics ethnicity classifications, where it is sub-divided into 'Caribbean', 'African' and 'Other Black' groups.

King Arthur is a legendary British leader of the late 5th and early 6th centuries, who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the early 6th century. The details of Arthur's story are mainly composed of folklore and literary invention, and his historical existence is debated and disputed by modern historians. The sparse historical background of Arthur is gleaned from various sources, including the Annales Cambriae, the Historia Brittonum, and the writings of Gildas. Arthur's name also occurs in early poetic sources such as Y Gododdin.

The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). Some Welsh and Breton tales and poems relating the story of Arthur date from earlier than this work; in these works, Arthur appears either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. How much of Geoffrey's Historia (completed in 1138) was adapted from such earlier sources, rather than invented by Geoffrey himself, is unknown.

Black Irish is an ambiguous term used mainly outside of Ireland.^{[citation needed]} Over the course of history, and in different parts of the Irish diaspora throughout the world, it has been subject to several different although somewhat overlapping meanings, encompassing physical appearance, religious affiliation, ethnicity, subculture and poverty. Modern traditionalists, however, maintain the term to be synonymous with a dark-haired phenotype exhibited by certain individuals originally descended from Ireland. Opinions vary in regard to what is perceived as the usual physical characteristics of the so-called Black Irish: e.g., dark hair, brown eyes and medium skin tone; or dark hair, blue or green eyes and fair skin tone. Unbeknown to some who have used this term at one time or another^{[citation needed]}, dark hair in people of Irish descent is extremely common, although darker skin complexions appear less frequently.

Early 21st century genetic studies have provided new insights into the origins of Irish people as well as their neighbours in the British Isles. Correspondingly, researchers in the field have suggested that migrations from Prehistoric Iberia (Spain, Portugal and also the Basque region) can be viewed as the primary source for their genetic material,^{[citation needed]} having demonstrated marked similarities with modern representatives of the aforementioned time period in that of the Basque people. However, the majority of Irish males fall under the R1b sub-clade L-21, which is quite rare for Basques.

The term Anglo-Saxon is used by some historians to designate the Germanic tribes who invaded and settled the south and east of Britain beginning in the early 5th century and the period from their creation of the English nation up to the Norman conquest. The Anglo-Saxon era denotes the period of English history between about 550 and 1066. The term is also used for the language, now known as Old English, that was spoken and written by the Anglo-Saxons and their descendants in England (and part of southeastern Scotland) between at least the mid-5th century and the mid-12th century.

The Benedictine monk Bede, writing in the early 8th century, identified the English as the descendants of three Germanic tribes:

Their language, Old English, which derived from Ingvaeonic West Germanic dialects, transformed into Middle English from the 11th century. Old English was divided into four main dialects: West Saxon, Mercian, Northumbrian and Kentish.

The term Anglo-Saxon can be found in documents produced in the time of Alfred the Great, who seems to have frequently used the titles rex Anglorum Saxonum and rex Angul-Saxonum (king of the English Saxons). The Old English terms ænglisc ('Angle-kin') and Angelcynn ('gens Anglorum') had already lost their original sense of referring to the Angles, as distinct from the Saxons, when they are first attested. In their earliest sense they referred to the nation of Germanic peoples who settled eastern Britain from the 5th century.^{[citation needed]} The indigenous Britons, who wrote in both Latin and Welsh, referred to these invaders as 'Saxones' or 'Saeson' – the word Saeson is the modern Welsh word for 'English people'; the equivalent word in Scottish Gaelic is Sasannach and in the Irish language, Sasanach.