Any of a number of powerful automobiles with front engines and rear wheel drive produced from the early '60s until the present. To be considered a true muscle car requires around at least a 1hp/10lbs ratio, generally speaking. Some lighter muscle cars - especially those from the late 60's - are better described as 'Ponycars', because they were very compact and light, similar in design to Ford's Mustang. There were a lot of very powerful but heavy musclecars, and debate exists over which one was the 'first'. Some would say that the old Chrysler 300 qualifies, while others say that the Pontiac GTO was the first true musclecar. The Australians even had a few 6-cylinder cars with blistering performance back in the 60's that also qualify, so the term 'muscle car' doesn't just just refer to V-8 vehicles. Regardless, most people would not consider any 4-cylinder high performance vehicle to be a muscle car; they are more accurately described as sports cars usually.
1964 Pontiac GTO, 1967 Mercury Cougar, 1969 Javelin, 1969 Chevrolet Chevelle, 1967 Dodge Charger
- note - There were many different engines available for these cars. Some of those engines lacked the performance to make even these specific models 'muscle cars'. A stock 6-cylinder Chevelle would not be considered a muscle car, for example.