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Partial Quote from the referenced below Wikipedia page.
Prius is a Latin word meaning "before". According to Toyota, the name was chosen because the Prius was launched before environmental awareness became a mainstream social issue. On February 20, 2011 Toyota announced that "Prii" was the most popular choice, and the new official plural designation.
The Toyota Prius is a full hybrid electric mid-size hatchback vehicle developed and manufactured by Toyota in Japan. The Environmental Protection (EPA) rate the Prius as among the cleanest vehicles sold in the United States based on smog-forming emissions. The Prius first went on sale in Japan in 1997, and was available all four Toyota Japan dealerships, making it the first mass-produced hybrid vehicle. It was subsequently introduced worldwide in 2000. The Prius family of cars reached global cumulative sales of 3.67 million units through March 2013, representing 71.6% of Toyota hybrid sales of 5.12 million cars.
The U.S. 2012 model year includes Toyota's Vehicle Proximity Notification System (VPNS), which is designed to alert pedestrians, the blind, and others of the vehicle's presence due to significant noise reduction typical of a hybrid vehicle traveling at low speeds in all-electric mode
Read more:
http://en.wikipedia.org/wiki/Toyota_Prius
In this panorama you may see the Gasoline/ Petrol engine on the back left and the Electric Traction Motor lower right corner of the engine compartment.