This straight lift off from Wikipedia defines an SUV pretty well.
A sport utility vehicle (SUV) is a generic marketing term for a vehicle similar to a station wagon, but built on a light-truck chassis.
It is usually equipped with four-wheel drive for on- or off-road ability, and with some pretension or ability to be used as an off-road vehicle.
Not all four-wheel drive vehicles are termed as SUV.
Some SUVs include the towing capacity of a pickup truck with the passenger-carrying space of a minivan or large sedan.
Personally, I would define SUVs as a large vehicle that has a relatively high GC with or without 4wd and can carry at least 5 adults comfortable along with a reasonable amount of luggage. A light truck chassis makes it more rugged for off road terrain.