For the last three generations of the Toyota Prius' existence, Toyota has been curiously tight-lipped about the official plural form of the car's name. This has led to years of confusion, as journalists, dealers, and Prius enthusiasts--yes, they exist--coined all sorts of awkward terms, such as the difficult to say Priuses.
With the unveiling of the Prius V and the Prius C concept and the automaker's plans to expand Prius into a hybrid sub-brand, Toyota, for the first time, was forced to officially choose a plural form of Prius. So, the automaker crowdsourced it, running a contest on its Prius family Web site and on Facebook to decide the official plural form. Users could choose from one of five options: Prius, Priuses, Prii, Prium, and Prien.
This week, the automaker and the people have finally weighed in and come to a consensus. The official plural form of Prius is Prii, as in, "Peter passed a pack of parked Prii." Personally, I'm sticking with my personal plurality: Priopotami.