There will never be a correct definition. Different regions, different age groups, different races, and even the different genders have different names for even things we use or eat every day.
My wife's parents are from the northwest, I had no Idea a "rompus room" is what we in the south call a "bonus room".
Soda, pop, coke, soda pop is another big one, as is sub, hoagie, foot long, grinder, etc.
As long as there are people of different ages, races, regions, and genders there will never be a fully agreed upon definition. There are some crazy variations in what is a "rat rod" or a "hot rod" as well. Some people think classic cars should ONLY be anything 1972 and older and others anything that's 20 or 25+ years old is a classic (sounds crazy to think of a 93 camry as a classic, huh?)
So what is a pony car? A muscle car? Damn fun to drive, that's what