According to Muscle Cars, a book written by Peter Henshaw, a "muscle car" is "a product of the American car industry adhering to the hot rodder's philosophy of taking a small car and putting a large-displacement engine in it. The Muscle Car is Charles Atlas kicking sand in the face of the 98 horsepower weakling." Henshaw further asserts that the muscle car was designed for straight-line speed, and did not have the "sophisticated chassis", "engineering integrity", or "lithe appearance" of European high-performance cars.
In the United States, lightweight cars featuring high-performance engines were termed "supercar" before the classification of muscle car became popular. For example, the 1957 Rebel's "potent mill turned the lightweight Rambler into a veritable supercar." "From the mid-sixties to the mid-seventies, what we now think of as muscle cars were more commonly called 'Supercars,' often (though not always) spelled with a capital S." This term described the "dragstrip bred" affordable mid-size cars of the 1960s and early 1970s that were equipped with large, powerful V8 engines and rear-wheel-drive. "In 1966, the supercar became an official industry trend" as the four domestic automakers "needed to cash in on the supercar market" with eye-catching, heart-stopping cars. Examples of the use of the supercar description for the early muscle models include the May 1965 Car Life road test of the Pontiac GTO along with how "Hurst puts American Motors into the Supercar club with the 390 Rogue" (the SC/Rambler) to fight in "the Supercar street racer gang" market segment. Moreover, the "SC" in the model name stood for "SuperCar".
The supercar market segment in the U.S. at the time included special versions of regular production models that were positioned in several sizes and market segments (such as the "economy supercar"), as well as limited edition, documented dealer-converted vehicles. However, the supercar term by that time "had been diluted and branded with a meaning that did not respect the unique qualities of the 'muscle car'."
Opinions vary as to whether high-performance full-size cars, compacts, and pony car qualify as muscle cars.