Prydain is the modern Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

 name for Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

.

Medieval

Prydain is the medieval Welsh term for the island of Britain (the name Albion

Albion

Albion is the oldest known name of the island of Great Britain. Today, it is still sometimes used poetically to refer to the island or England in particular. It is also the basis of the Scottish Gaelic name for Scotland, Alba...

 was not used by the Welsh). More specifically, Prydain may refer to the Brittonic

Britons (historical)

The Britons were the Celtic people culturally dominating Great Britain from the Iron Age through the Early Middle Ages. They spoke the Insular Celtic language known as British or Brythonic...

 parts of the island; that is, the parts south of Caledonia

Caledonia

Caledonia is the Latinised form and name given by the Romans to the land in today's Scotland north of their province of Britannia, beyond the frontier of their empire...

. This distinction appears to derive from Roman

Ancient Rome

Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 times, when the island was divided into Roman Britain

Roman Britain

Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...

 to the south and the land of the Caledonians

Caledonians

The Caledonians , or Caledonian Confederacy, is a name given by historians to a group of indigenous peoples of what is now Scotland during the Iron Age and Roman eras. The Romans referred to their territory as Caledonia and initially included them as Britons, but later distinguished as the Picts...

 to the North. The peoples north of the Roman borders eventually came to be known as the Picts

Picts

The Picts were a group of Late Iron Age and Early Mediaeval people living in what is now eastern and northern Scotland. There is an association with the distribution of brochs, place names beginning 'Pit-', for instance Pitlochry, and Pictish stones. They are recorded from before the Roman conquest...

 (Welsh: Brithwyr); the Welsh term for Pictland was Prydyn, which caused some confusion in the texts with Prydain.

In Middle Welsh texts, the related term Ynys Prydein (Island of Britain), or Ynys Brydein, can also refer to the island (ynys) itself but more often is a name for the Brittonic territories south of Caledonia. It is in this context that the name of the collection of traditional material arranged in triads known as Trioedd Ynys Prydein

Welsh Triads

The Welsh Triads are a group of related texts in medieval manuscripts which preserve fragments of Welsh folklore, mythology and traditional history in groups of three. The triad is a rhetorical form whereby objects are grouped together in threes, with a heading indicating the point of likeness...

should be understood. In modern Welsh ynys means 'island', but in Middle Welsh it can also mean 'land' or 'realm' (cf. Latin insula).

There are numerous other instances of the term Prydain in medieval Welsh texts. One of the best known is found in the title of the 10th century vaticinatory poem Armes Prydein

Armes Prydein

Armes Prydein is an early 10th-century Welsh prophetic poem from the Book of Taliesin.In a rousing style characteristic of Welsh heroic poetry, it describes a future where all of Brythonic peoples are allied with the Scots, the Irish, and the Vikings of Dublin under Welsh leadership, and together...

('The Prophecy of Britain').