Healing usually refers to mutants with the ability to heal themselves at accelerated rates. Because of this speedy recovery, they can withstand more physical injury than those who heal at a normal rate. This ability is often referred to as self-healing or regeneration.
Some self-healers also have immunities to drugs, alcohol, medications, illnesses and other foreign elements due to accelerated metabolisms, healing rates, etc.
However, some healers are able to heal others, sometimes at the cost of being able to heal themselves.