Aside from the traditional definition, a mutant is a person who has a gene that's mutated in a way that allows us to naturally develop superhuman powers and abilities. You know I'm a telepath, I've met a shapeshifter, someone who can release kinetic energy, another young man whose cells could change to adapt to his surroundings. There's no pattern we've established just yet as for why one person develops an ability versus another, but we are all genetically different from humans and similar to each other.