Oskar Sommars' term for what is now called the
Lancelot-Grail Cycle. Sommars was the first to edit the five-part romance into a modern typeface, published in 1907. However, this seven-volume work is extremely difficult to find; copies go for $1500.
The name derives from the fact that the romance was written in the "vulgar" language; of course, almost all romance is written in a "vulgar" language--that is why it's called romance.
See also Lancelot-Grail Cycle.