A mutant is someone who is born with unique powers that do not manifest until they are older, they are born with

The X-Men

hidden powers.  Their powers can also be obtained if they are in a state of anger and rage, if someone they love is killed.  For example, Magneto's powers did not manifest to their full potential until his mother was shot and killed.  Wolverine's claws did not emerge from his skin until his father was murdered.  Not only can mutants be born with hidden powers but they can also emerge when they are first born.  Bobby Drake AKA Iceman's powers emerged when he was only an infant, he froze his milk bottle.  Some mutants appear human while others appear to be more animal like.  Mutants have been discriminated against throughout history because they are different from normal humans.   In the middle ages they were called witches and were burned at the stake.  The X-Men, founded by Professor Charles Xavier (Professor X), used his powers to protect humans even though humans hated him because he was different.  Magneto and his Brotherhood of Evil Mutants sought to eliminate humans and believed they were superior to them.  Every mutant has a choice, to use their powers for good or for evil.