Caledonia
Caledonia is a historic region essentially referring to Scotland.
History
Caledonia was an exonym used by the Romans for the northern territories beyond their control in the British Isles. The border was demarcated by Hadrian's Wall, built in 122. The Antonine Wall was built significantly further north in 142, and together with the River Forth demarcated a new border, but this was abandoned within two decades.
The Caledonians were a Celtic tribal society. They did not have a centralized state.
Around the time of the Roman withdrawal, the Caledonians came to be known by another exonym: Picti or Picts. Another name for Caledonia then is Pictland.
The dominant Caledonian kingdom was Fortriu. By the reign of Bridei I in the 6th century they had established court in Inverness.
Caledonia at this time was within the sphere of influence of the Anglian kingdom of Northumbria to the south. The Gaelic kingdom of Dál Riata, also a tributary of Northumbria, had begun to expand from Ireland into the western lowlands. Then in the 7th century, Bridei II defeated Ecgfrith of Northumbria, leading to a period known as the Verturian hegemony. Dál Riata now fell under Caledonian influence.
Onuist I achieved the greatest extent of Fortriu in the 8th century. He conquered the entirety of Dál Riata and briefly managed to hold it, and raided deep into Northumbria (though without any effort to hold the territory).
Over time Fortriu Gaelicized, likely a result of the Gaelic Kenneth I coming to power. By the 9th century it and Dál Riata had merged to become the kingdom of Alba.