Trying to use a generic
Trying to use a generic dictionary definition to describe a Volt limits us. Any joining of two technologies could be called a hybrid according to the dictionary. I have hybrid golf clubs...
The fact that the Volt is designed to operate as a fully functioning EV for 40 miles at all speeds does justify using "EV" in the name/description. Many BEV's, especially ones made with EV kits, have an EV range less than 40 miles.
The Prius Plugin Hybrid, by design, uses the gas engine any time the situation calls for it. The PIP is not designed to operate exclusively as an EV at all speeds. It is designed to act as a hybrid even when the battery is fully charged. This is a very big distinction, and why the Prius Plugin Hybrid is appropriately called a hybrid, and why the Volt is called an EREV.