Just depends on your point of view. Mine is from running a race sanctionning body for a long time. I'm with purp... I mean
@Mechanogeek for the stock category. I would even had to say that todd description fit the stock category but for the move to 15 inches wheels.
Some are also using the term "street" for stock.
At the other end of the Spectrum, Modified usually describe car with engine swaps from another manufacturer (K20 would fit that but a Fiat DOHC into a x10 would be into a gray area) or major body changes (Dallara fenders, or windshield removal) or empty interior.
In the middle, different sanctioning bodies use different approaches with different names: super-stock, street prepared, street modified, prepared, etc. I remember there was a time with points preparation system in Canada (less than 6 for ss, less than 15 for prepared,...).
I have a friend (I will not tell his name) with a green Kermit car who once told me he wanted to keep his car STOCK. Then, later he explained he was not doing any modifications, just ameliorations. Car appear stock but WE would know the bumpers are not correct for the year, there were no 15" wheels ever on factory X1/9's and hidden in the engine, the cam is no longer stock. So, sometimes, we just evolved, sometimes our description of stock is changing.
So, in the end, for me a stock car is like @mechanogeed says "nothing changed except expendable parts, like tires, brake pads, hoses and belts, fluids and filters. Wear parts like bushings, pads, belts and etc replaced with same material replacements". In a slighty upgrade caterogy, I would accept any changes that could be revert: 15 inches wheels, brake upgrades, race seat, even twin carbs and headers... because the car could return to his "as deliver from the factory" set-up. But as soon as you start cutting some metal, opening the head flow, bolting camber plates, you are in a "street modified/street prepared" category.