A word whose meaning is often the subject of debate.
When Lady Charlotte Guest decided to translate tales from the Red Book of Hergest and White Book of Rhydderch, she saw that what are traditionally the first four stories--"Pwyll pendeuc Dyfed," "Branwen uerch Llyr," "Manawyddan fab Llyr," and "Math fab Mathonwy," all end "And here ends this branch of the mabinogi"--except for "Pwyll," which ends "branch of the mabinogion." This was a scribal error, as there is no word "mabinogion" in Welsh. She assumed that "mabinogion" is plural for "mabinogi," and thus named her book The Mabinogion.
But what is a mabinogi?
Guest assumed that a "mabinogi" was a story for children, as "mab" or "mabon" means "boy." However, these were not stories for children; they were recorded in several manuscripts for adults. So they certainly weren't tales for children. However, the name stuck, and so we have always called this collection of tales The Mabinogion.
That doesn't answer my question.
There is the Welsh god Mabon ap Modron, who appears in one of the tales--"Culhwch ac Olwen." He is the sun god who must be released from prision. (see w/u).
The Four Branches seem to revolve around the Children of Llyr and the Children of Don; more specifically, they revolve around Pryderi, the son of Pwyll, nephew of Branwen, step-son of Manawyddan, and rival of Math's nephew Gwydion, who eventually kills him. If one examines what is known of Mabon ap Modron and his Irish counterpart Oengus mac ind Og (see both w/u's for more), and compares it to the life of Pryderi, it becomes evident that Pryderi is another form of Mabon.
This has led scholars to conclude that "The Four Branches of the Mabinogi" refer to the Four Stories about Mabon and his Family.
And so, while my Welsh dictionary insists that a "mabinogi" is a "fairy tale," it most certainly is not.