This is the story which Lady Guest called Pwyll Prince of Dyfed. This First Branch is followed by a Second Branch, Third Branch and Fourth Branch, which Lady Guest called Branwen the Daughter of Llyr; Manawyddan the son of Llyr; and Math the son of Mathonwy.
The description of each of these stories as ‘a branch of the Mabinogi’ is to be found at the end of the original Welsh versions. A branch means a section, and mabinogi is an archaic Welsh word, similar to the Latin infantia or the French enfance. In different contexts, it may have four meanings: a period of youth; a story about a period of youth; a story about a hero; or, simply, a story. Here, it is thought that the stories may relate to the period of youth of the character known as Pryderi, even though his role is an extremely peripheral one in the Second and Fourth Branch.
At the end of one of the original Welsh versions of the First Branch, the word mabinogi is incorrectly written as mabynnogyon (or mabinogion in modern spelling). Lady Charlotte Guest did not realise this was an error. She assumed that mabinogion was the plural of mabinogi and chose to adopt it as her title for the collection as a whole. In fact, the word mabinogion does not intentionally appear in any of the stories. Moreover, the word mabinogi is, strictly speaking, only relevant to the first four stories known as the ‘Branches of the Mabinogi’. Its adoption by Lady Guest in a so-called plural form and as a title suitable for the twelve stories in her collection was, therefore, based on a misconception. However, because the name Mabinogion has become so strongly identified in English with the collection as a whole, it has been retained by translators, literary critics and readers.