Power Nullification, or Negation, is the ability to simply nullify the powers and abilities of others, negating their effects. The mechanisms by which this effect is accomplished and their restrictions vary from character to character, but the ability is often limited to a certain type of power, such as supernatural phenomena in general, or magic. In some cases, an ability can only be negated once it's been used or activated, but in others, powers can be nullified before they're even utilized. The effect may be temporary or permanent, and may be restricted by touch, or be something that the user can apply to any ability they see, or within a field that negates any powers within.
Some characters accomplish these effects through applications of other abilities rather than a standalone ability in itself, such as using Reality Warping or Power Absorption to weaken or remove their abilities.
It would be a No Limits Fallacy to assume someone with this ability can nullify anything that falls within the phenomena their powers are made to counter. It is to be assumed that any negation power has its limitations based on the strongest thing it has nullified, and a character may be resistant to the ability (at least in one of its forms), preventing it from having its usual effects.
However, certain forms of Power Nullification may prevent the usage of powers through less direct methods, making resistance less useful and the strength of the ability less relevant.
Anti-Magic: Abilities that focus on nullifying magic, potentially even rejecting physical characteristics augmented by magic or making it so that the user cannot be harmed by magic at all. This can negate spells, from simple blasts to more complicated effects, and may be capable of stripping magical items of their enchantments and magical properties.
Hax Negation: The User can negate an opponent's ability to use their abilities or has a physiology that is able to negate specific abilities being used at all, or it could be that their abilities are designed to specifically counter abilities used against them.
Acausality Negation: The User can bypass and or negate specific forms of Acausality with their physiology or with specific abilities and subsequently negate the resistance provided by that type of Acausality.
Immortality / Regeneration Negation: Many characters and weapons have the ability to ignore the immortality and regenerative abilities of their opponents, allowing them to kill characters with wounds they would otherwise survive. As with other powers, this is generally limited by the strongest level of regeneration and the types of immortality that the ability has nullified, and an ability may be limited to just immortality or just regeneration. Killing a character through means they can't survive even with their immortality doesn't count; killing someone who can regenerate from just a soul can just as easily be Soul Manipulation, depending on the context.