Caledonia is the Latin name given by the Romans to the land in today's Scotland north of their province of Britannia, beyond the frontier of their empire. The etymology of the name is probably from a P-Celtic source. Its modern usage is as a romantic or poetic name for Scotland as a whole, comparable with Hibernia for Ireland and Cambria for Wales.
The original use of the name, by Tacitus, Ptolemy, Lucan and Pliny the Elder, referred to the area (or parts of the area) later known as Pictavia or Pictland north of Hadrian's Wall in today's Scotland. The name may be related to that of a large central Pictish tribe, the Caledonii, one amongst several in the area and perhaps the dominant tribe, which would explain the binomial Caledonia/Caledonii.
The name of the Caledonians may be found in toponymy, such as Dùn Chailleann, the Scottish Gaelic word for the town of Dunkeld meaning "fort of the Caledonii", and possibly in that of the mountain Sìdh Chailleann, the "fairy hill of the Caledonians".
According to Historia Brittonum the site of the seventh battle of the mythical Arthur was a forest in what is now Scotland, called Coit Celidon in early Welsh.
According to Zimmer (2006), Caledonia is derived from the tribal name Caledones (or Calīdones), which he etymologizes as "'possessing hard feet', alluding to standfastness or endurance", from the Proto-Celtic roots *kal- "hard" and *φēdo- "foot". Similarly, Moffat (2005) suggests the name is related to the Welsh word caled, "hard", which could refer to the rocky land or the hardiness of the people. Keay and Keay (1994) state that the word is "apparently pre-Celtic".
The exact location of what the Romans called Caledonia in the early stages of Britannia is uncertain, and the boundaries are unlikely to have been fixed until the building of Hadrian's Wall. From then onwards, Caledonia stood to the north of the wall, and to the south was the Roman province of Britannia (consisting of most of what is now England and Wales). During the brief Roman military incursions into central and northern Scotland, the Scottish Lowlands were indeed absorbed into the province of Britannia, and the name was also used by the Romans, prior to their conquest of the southern and central parts of the island, to refer to the whole island of Great Britain. Once the Romans had built a second wall further to the north (the Antonine Wall) and their garrisons advanced north likewise, the developing Roman-Britons south of the wall had trade relations with the Picts north of the wall, as testified by archaeological evidence, much of it available at the National Museum of Scotland in Edinburgh.
The modern use of "Caledonia" in English and Scots is either as a historical description of northern Britain during the Roman era or as a romantic or poetic name for Scotland as a whole.
The name has been widely used by organisations and commercial entities. Notable examples include Glasgow Caledonian University, ferry operator Caledonian MacBrayne, and the now-defunct British Caledonian airline. The Caledonian Sleeper is an overnight train service from London to Scottish destinations. In music, "Caledonia" is a popular folk ballad written by Dougie MacLean in 1977 and published in 1979 on an album of the same name; it has since been covered by various other artists, including Amy Macdonald. The web series Caledonia and associated novel is a supernatural police drama that takes place in Glasgow, Scotland.
Ptolemy's account also referred to the Caledonia Silva, an idea still recalled in the modern expression "Caledonian Forest", although the woods are much reduced in size since Roman times.
Some scholars point out that the name "Scotland" is ultimately derived from Scotia, a Latin term first used for Ireland (also called Hibernia by the Romans) and later for Scotland, the Scoti peoples having originated in Ireland and resettled in Scotland. Another, post-conquest, Roman name for the island of Great Britain was Albion, which is cognate with the Scottish Gaelic name for Scotland: Alba.