RE Mutants defined:
According to the official Marvel Super Heroes RPG, Mutants are those born with powers as a genetic mutation.
Others, like Spiderman or the Hulk, are defined as “altered humans”
From wikipedia, which in this case I concur with:
“Each character had an origin, which put ceilings on a character’s abilities and superpowers. The origins included: Altered Humans (normal people who acquired powers, such as Spider-Man or the Fantastic Four), High-Tech Wonders (normal people whose powers come from devices, e.g., Iron Man), Mutants (persons born with superpowers, such as the X-Men), Robots (created beings such as the Vision and Ultron), and Aliens (a blanket term used to cover non-humans, including extra-dimensional beings such as Thor and Hercules).”