I don't really like the term hybrid at all it doesn't tell you anything about a car other than it's not standard. It's too broadly used to mean anything in particular, hence Paul's interpretation above.
But I'd be careful to keep dual fuel in a separate category. I wouldn't like to call an LPG car a hybrid, after all I don't consider it a very green solution. (lower calorific value means you burn twice as much giving same overall energy per carbon emmissions, and its fossil not renewable) but a dual fuel electric petrol is greener...
if you look at a Prius it's primary fuel is petrol but, it does technically have a second fuel source, and just because you don't fill it up doesn't mean its not a fuel (just as you don't fill up a solar car) but by exploiting something that all other cars give off to the environment, that being regeneration from the braking energy, there is a second energy store available to the drive-train. albeit a very minor amount in comparison with other duel fuels.
so I guess the question becomes is self generating/collecting/accumulating fuel a second fuel source?