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, Highland Gaelic or Gàidhlig, to avoid confusion with the other 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 the Scots language (aka Lowland Scots), which is an Anglic language descended from early Middle English. Within Scotland, Gaelic is pronounced ['ga?l?k] while outside Scotland it is usually pronounced /'ge?l?k/.

Scottish 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 also replaced English in considerable areas. 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 Rhinns of Galloway by the 5th or 6th century.

The Gaelic language eventually displaced Pictish north of the Forth, and until the late 15th century it was known in English as Scottis. Gaelic began to decline in mainland Scotland by the beginning of the 13th century, and with this went a decline in its status as a national language. By the beginning of the 15th century, the highland-lowland line was beginning to emerge.

By the early 16th century, English speakers gave the Gaelic language the name Erse (meaning Irish) and thereafter it was invariably the collection of Middle English dialects spoken within the Kingdom of the Scots that they referred to as Scottis (whence Scots). This was ironic as it was at this time that Gaelic was developing its distinctly Scottish forms characteristic of the Modern period. Nevertheless, Gaelic has never been entirely displaced of national language status, and is still recognised by many Scots, whether or not they speak Gaelic, as being a crucial part of the nation's culture. Others may view it primarily as a regional language of the highlands and islands.

Gaelic has a rich oral (beul-aithris) and written tradition, having been the language of the bardic culture of the Highland clans for several centuries. The language preserved knowledge of and adherence to pre-feudal laws and customs (as represented, for example, by the expressions tuatha and dùthchas). The language suffered especially as Highlanders and their traditions were persecuted after the Battle of Culloden in 1746, and during the Highland Clearances, but pre-feudal attitudes were still evident in the complaints and claims of the Highland Land League of the late 19th century: this political movement was successful in getting members elected to the Parliament of the United Kingdom. The Land League was dissipated as a parliamentary force by the 1886 Crofters' Act and by the way the Liberal Party was seen to become supportive of Land League objectives.