The vessel from which our Saviour drank at the Last Supper, and which (as it is said) was afterwards filled by Joseph of Arimathe'a with the blood that flowed from His wounds. This blood was reported to have the power of prolonging life and preserving chastity. The quest of this cup forms the most fertile source of adventures to the knights of the Round Table. The story of the Sangreal or Sangraal was first written in verse by Chrestien de Troyes (end of the tenth century), thence Latinised (thirteenth century), and finally turned into French prose by Gautier Map, by “order of Lord Henry” (Henry III.). It commences with the genealogy of our Saviour, and details the whole Gospel history; but the prose romance begins with Joseph of Arimathe'a. Its quest is continued in Percival, a romance of the fifteenth century, which gives the adventures of a young Welshman, raw and inexperienced, but admitted to knighthood. At his death the sangreal, the sacred lance, and the silver trencher were carried up to heaven in the presence of attendants, and have never since been seen on earth.
Tennyson has a poem entitled The Holy Grail.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894