By your definition a 1915 Cadillac is a muscle car.

 

Im speaking of late model cars. I dont think a car has to be really fast to be a muscle car. However the potential definately should be there.

 

An AMC Hornet or Matador? The Buick Regal with the 305? or more modern: A BMW 7 series or the latest Lexus Q sedan or droptop?

Simply having a V8 and RWD doesn't automaticlly make a muscle car. A Pontiac GTP coupe has FWD and a blown V6, and one could make a very good arguement for inclusion into muscle car ranks. It's big, it's actually quicker than all but the top model GTOs of the 60s, and it's very receptive to performance add-ons.

As for the "head turning" requirement, no muscle car ever made had a specific "head turning" quality, since all were build on run of the mill "boring" midsize family cars. Beyond a hood scoop, rims, black-out grill, and the occasional rear spoiler, you'll never tell one from a base model. Best modern example is the Impala SS vs Caprice. Another example that's just as accurate (though not as quick) is the Maurader vs Grand Marquis.

Thunderbird isn't a muscle car. The MN-12 Supercoupe Thunderbirds were (even though they had a supercharged V6). But the current Thunderbird isn't by a very long shot.

Last but not least, muscle cars don't have 2 seats.