Immortal is the term given to people who won't die naturally. Immortality is a common power in fiction featuring supernatural beings such as Gods, werewolves, vampires etc. Others can be gifted with it via mutation, cosmic powers etc. There are several variations of immortality with various ranges of usefulness:
1: Longevity: Longevity is the power to live a long time, people gifted with this type of immortality cannot die from natural causes, old age, illnesses etc, but can be killed by unnatural causes.
2: Immortality without regeneration: This type of immortality is not useful for combat. Essentially bodily harm cannot kill you but your original form can be mutilated. An immortal of this category can be maimed and rendered useless in battle. If the character in question has high durability then this type of immortality can be more useful.
3: Immortality via regeneration: You can maim the person, kill him or her, etc. and would still still come back. This type of immortality hinges on how well the character can regenerate. Most high tier regeneration grants immortality.
4: Immortality via Godhood or protection from a deity: This kind of immortality is granted by a God or is given by virtue of Godhood. Only a God of higher status can override it.
5: Perfect Immortality: Complete and utter inability to ever die. This is typically reserved for questionable omnipotents and cosmic characters who are strong enough that dying isn't exactly high on their list of worries, even if they were capable of it.
6: Parasitic: The person attains a sort of immortality by bodyhopping, transferring their soul to another body.
7: Undead: Self explanatory. The undead generally double up with other types of immortality. Zombies may possess similar resistances to the "Immortality Only" category. Ghosts occasionally double up with "Perfect Immortality" as they can not usually be killed again.