The official plural of Prius when referring to the car is Prii. In Latin, the plural of the word pruis is priora.
The Toyota Prius was given its name because the meaning of the Latin word "prius" means "before," and the environmentally friendly car was introduced before eco-friendly ideals had become mainstream. In an effort to engage its consumers, Toyota asked for the public's help in designating an official plural word to be used when referring to the automobile. Suggestions that came in included Prien, Priuses, Prii, Prius and Prium. On February 20, 2011, Toyota announced that Prii was the most popular choice and gave it the official designation.