Prydain is the medieval Welsh term for the island of Britain (the name Albion was not used by the Welsh). More specifically, Prydain may refer to the Brittonic parts of the island; that is, the parts south of Caledonia. This distinction appears to derive from Roman times, when the island was divided into Roman Britain to the south and the land of the Caledonians to the North. The peoples north of the Roman borders eventually came to be known as the Picts (Welsh: Brithwyr); the Welsh term for Pictland was Prydyn, which caused some confusion in the texts with Prydain.
Source: Prydain – Wikipedia