An individual with slightly different DNA (a mutation), either by natural occurrence or by an external process (esp. genetic engineering), which imparts a new characteristic to the individual.
A mutant is an individual with whose DNA differs from the rest of their species.
This difference often imparts a beneficial super-human ability, or, less often, a disadvantageous ability or physical disfigurement.
One of the best-known examples of mutants are Marvel's X-Men.