Scottish Gaelic (Gàidhlig) is a member of the Goidelic branch of Celtic languages. This branch also includes the Irish and Manx languages. It is distinct from the Brythonic branch of the Celtic languages, which includes Welsh, Cornish, and Breton. Scottish, Manx and Irish Gaelic are all descended from Old Irish. The language is often described as Scottish Gaelic, Scots Gaelic, or Gàidhlig to avoid confusion with the other two Goidelic languages. Outside Scotland, it is occasionally also called Scottish, a usage dating back over 1,500 years; for example Old English Scottas. Scottish Gaelic should not be confused with Scots, because since the 16th century the word Scots has by-and-large been used to describe the Lowland Anglic language, which developed from the northern form of early Middle English. In Scottish English, Gaelic is /ˈɡælɪk/; outside Scotland, it is usually /ˈɡeɪlɪk/.
Gaelic, a descendant of the Goidelic branch of Celtic and closely related to Irish, is the traditional language of the Scotti or Gaels, and became the historical language of the majority of Scotland after it replaced Cumbric, Pictish and Old Norse. It is not clear how long Gaelic has been spoken in what is now Scotland; it has lately been proposed that it was spoken in Argyll before the Roman period, but no consensus has been reached on this question. However, the consolidation of the kingdom of Dál Riata around the 4th century, linking the ancient province of Ulster in the north of Ireland and western Scotland, accelerated the expansion of Gaelic, as did the success of the Gaelic-speaking church establishment. Placename evidence shows that Gaelic was spoken in the Rhins of Galloway by the 5th or 6th century.