Not having a HSD vehicle (or any CVT for that matter), I never thought of it much.

A planetary system with 3 independent and driven in/ouputs, one for the sun, one for the carrier, one for the ring, is DEFINITELY capable of providing a continuously variable gear ratio by varying the relative speeds of the three parts.

As Bob summarized, MG1 more or less dictates where the engine power goes, and in what proportion. Because there is no fixed, linear relationship between engine rpm and output rpm, globally speaking it is definitely CVT. If MG1 was not capable of changing speed to affect the motion of the engine/carrier relative to the ring/output, this would be a fixed reduction.

The gears themselves are fixed; it is the presence of the ability to change their relative rotation rates that makes it useful. The "transmission" is the entire system, not just the gears; therefore the "transmisison" is a CVT. Similarly, a normal transmisison includes all the hydraulic channeling required to engage clutches, all the idlers, etc.

Nowhere in the "definition" of CVT does it require that the mechanical parts be solely responsible for the ratio changes. Honda does it by sliding the belt up and down the conical pulley to affect the ratio; Toyota does it by using an electric motor (MG1) to affect the ratio.

The lay versions:
http://auto.howstuffworks.com/hybrid-car2.htm
http://www.ecrostech.com/prius/origi...plitDevice.htm