I think it sprang from putting beefy touring wheels & tires on a mountain-bike. Then someone put together this idea of a bike with a looooong wheelbase, and touring wheels. Instead of the 26" wheels found on most mountain-bikes of the 1980's. This could somewhat define a 'hybrid' to someone who hasn't seen one, but is familiar with bicycles. You can, at least, visualize the concept from this description. But variation is the spice of life - so these "rules" can be modified at any time.

In my opinion, the Hybrid-Bicycles is a truly distinct form that is neither a mountain or road bike. It is a Hybrid.