adamantium hero, n.: a character so tough and resistant to injury that s/he is able to shrug off normally fatal wounds with little or no effect.
See adamantium. [GJ1]

Addendum: This isn't a bad thing in superhero games, but in genres where combat is supposed to sometimes end in death, it gets ridiculous. TSR and Palladium are notorious for games systems that encourage this, but it can be observed in some of the more over-the-top anime and Hollywood action movies. [GJ1]
Addendum #2: Most gaming systems are structured so one character can face several opponents at one time in a grand heroic tradition. Unfortunately, most rules set this up by allowing a character to absorb damage from all these opponents rather than by allowing them to avoid it. This results in a character immune to environmental hazards and who cannot be laid out in a single blow from a normal weapon, but must be beaten into unconsciousness by repeated blows or by emptying several clips of bullets into them. [MC]
Addendum #3: This is also why most RPG characters can't be assassinated -- a surreptitious dagger to the neck only does a few points of damage, unless special rules are allowed. [EDE]