Prydwen: PRID-wen
Welsh: "Fair Face"
1. The shield of King Arthur on which is painted a picture of the Virgin Mary, according to Geoffrey of Monmouth's History of the Kings of Britain. It is one of Arthur's three great weapons, the others being the sword Caliburn and the spear Ron.
2. Arthur's ship, according to The Mabinogion's "Culhwch ac Olwen," on which the expedition to Ireland is made.
Why one says it's a shield and the other a ship is beyond me.
UPDATE: as skow writes to me: "It's not that strange I think, maybe the answer lies in a more symbolic realm. A ship protects one as well as a shield."
UPDATE 2 (3/11/2008): rootbeer277 points to the word escutcheon, which shows how ships and shields can be confused when translating; and perhaps Geoffrey, working in a French framework, is to blame.