Muscle car is a description according to Merriam-Webster Dictionary that came to use in 1966 for "a group of American-made two-door sports coupes with powerful engines designed for high-performance driving." The Britannica Dictionary describes these as "an American-made two-door sports car with a powerful engine."

Although the term was unknown for another fifteen-plus years, General Motors is credited by some as introducing the first "intentional" muscle car in 1949, when it put its 303-cubic-inch (5 L) Rocket V8 from its full-sized luxury car 98 model into the considerably smaller and lighter Oldsmobile 88. The competition between American manufacturers started when Chrysler installed the 331 cu in (5.4 L) Chrysler Hemi engine in the mid-range Chrysler Saratoga in 1951 that was normally installed in the full-sized luxury sedan Chrysler New Yorker. In 1952 Ford's luxury brand Lincoln introduced the 317 cu in (5.2 L) Lincoln Y-Block V8 and the rivalry began, where the Lincoln Capri was entered in the Pan American Road Race in both 1952 and 1953, and taking first and second place in 1954. This was followed by both the Oldsmobile 88 and Chrysler Saratoga being raced in stock form at NASCAR races across the country.

By some accounts, the "muscle car" term proper was originally applied to mid-1960s and early 1970s special editions of mass-production cars which were designed for drag racing, though it shortly entered the general vocabulary through car magazines and automobile marketing and became used generically for "performance"-oriented street cars.

By some period definitions and perceptions, the term muscle car came to connote high performance at budget prices, where extremely powerful engines were put into relatively bare-bones intermediate cars at extremely affordable prices. This wave, exemplified by the 1968 Plymouth Road Runner and companion Dodge Super Bee, were meant to undercut more expensive, more stylish, and better-appointed cars by General Motors and Ford that had come to define the market, such as the Pontiac GTO (1964), 396 Chevrolet Chevelle (1965), 400 Buick Gran Sport (1965), 400 Oldsmobile 442 (1965), as well as 427 Mercury Comet Cyclone (1964) and 390 Mercury Cyclone (1966). The Dodge and Plymouth cars also continued the performance tradition started at Chrysler with the full-sized Chrysler 300L when production ended in 1965.

By some definitions – including those used by Car and Driver and Road and Track magazines cited below, pony cars such as the Ford Mustang, Chevrolet Camaro and the Plymouth Barracuda and their luxury companions Mercury Cougar, Pontiac Firebird and the Dodge Challenger in that large, influential, and lucrative 1960s-70s niche, could also qualify as "muscle cars" if outfitted with suitable high-performance equipment.