The name “Picts” covers a number of different groups of people who lived mainly in the north and eastern parts of Scotland including (until the Vikings came) the Orkneys. Like the Scots of Dal Riada, lists of Pictish “overkings” have to an extent been preserved through history. The Pictish kingdom which dominates the records is that of Fortriu (sometimes that word is used for all Picts). Where this kingdom was based is unclear. Initially it was taken to be around Perth, but it may actually have been much further north.
The name we use for them is Latin, first recorded in the late 3rd century and probably means “painted people[1].”

Over the centuries the Picts had been adversaries or allies with the other main groups mentioned here. In 685 they defeated the Northumbria Anglo-Saxons at the Battle of Dun Nechtain (also known as Nechtansmere) and this was an important moment in history. At the time the Northumbrians were probably the more powerful of the four groups mentioned here in the area but the outcome of this battle put a stop to all of that.
In the 7th century, the Picts may have had some control over Strathclyde when a Pictish king Nechtan (nothing to do with the battle) may also have been king of Strathclyde. By the middle of the 8th century there was certainly some joint rule between Dál Riata and the Picts. The Pictish King Óengus mac Fergus had control over both “kingdoms” and although this unity did not last, by the year 800, the king of the Picts, Caustantín mac Fergusa (who ruled from 793 to 820) was in a powerful enough position to ensure that his son, Domnall, would become king of Dál Riata, outliving his father by a further 15 years.
The idea of a “united kingdom” between Picts and Dalriatans was presumably around at the time and its momentum would only grow until the two groups eventually joined to become the kingdom of Alba – Scotland – later in the 9th century.
[1] For some of the earliest references, see https://archive.org/details/xiipanegyricila02baehgoog/page/n171/mode/2up , https://archive.org/details/L331AmmianusMarcellinusRomanHistoryIII2731ExcerptaValesiana/page/n61/mode/2up
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