In mediaeval texts (e.g. the Mabinogion), the term often refers to the northenmost part of the island, beyond the Forth and Clyde: where the island as a whole is meant, the phrase Ynys Prydain (Isle of Prydain) is commonly used. Compare Albion and Alba for a similar distinction.
The name has been used by a number of authors for ancient Celtic or fantasy lands, including numerous works of Stephen Lawhead, and Lloyd Alexander's Chronicles of Prydain.