to me it describes a make/model that is mechanically identical to a new one that are being sold at the dealership(doesn't necessarily have to be the same model year though).
I'm with db130 on this one. I would use that term for any model that is still for sale and hasn't yet undergone any major changes. I can buy a 2009 Mazda3 that is essentially the same as the one I have, so I still consider mine a "late model".
The ability to finance it makes sense. If you want a mechanical definition probably anything not using a carb, fuel injection of some sort along with a computer on engine.