To quote from wikipedia:
Absolute zero is the lowest possible temperature where nothing could be colder, and no heat energy remains in a substance. Absolute zero is the point at which molecules do not move (relative to the rest of the body) more than they are required to by a quantum mechanical effect called zero-point energy. It is a theoretical limit and cannot be achieved with the current technology available.
By international agreement, absolute zero is defined as precisely 0 K on the Kelvin scale, which is a thermodynamic (absolute) temperature scale, and −273.15 on the Celsius (centigrade) scale. Absolute zero is also precisely equivalent to 0 °R on the Rankine scale (also a thermodynamic temperature scale), and −459.67 degrees on the Fahrenheit scale. Though it is not possible to cool any substance to 0 K, scientists have made great advancements in achieving temperatures close to absolute zero, where matter exhibits quantum effects such as superconductivity and superfluidity. In 2000 the Helsinki University of Technology has reported reaching temperatures of 100 pK (1×10−10K).
So to sum it up, it's where everything is frozen, and it's not possible in real life. But just like FTL travel, that hasn't stopped authors of fiction. An Absolute Zero attack is generally considered to be a one - hit kill, capable of destroying most physical objects/creatures it contacts, regardless of their conventional durability.
Some ways to resist it, though, are:
- Able to regenerate if even a tiny part of your body is left (if all of it is hit by the AZ attack though, this won't work)
- Being able to regenerate from nothing/other dimensions
- Being non-corporeal
- Controlling matter and energy at a quantum level
- Having so much energy that your molecules can't be frozen (of course this depends on the power behind the AZ attack and its mechanism)
- Simply being tough enough to shrug it off (again, impossible according to real physics, but that never stopped fiction)
There are also some fictional attacks that are claimed to freeze things to below Absolute Zero. This would mean halting all movement even at a quantum level, which is even more impossible than conventional AZ, and would probably destroy the universe if it was possible. But that's fiction for you.
Examples of characters/entities that can use Absolute Zero attacks/powers:
- Akira from Samurai Deeper Kyo via his Inferno Chill attack (claimed to go below Absolute Zero)
- Alice from Project ARMS
- Buster Machine Quatre-Vingt-Dix from Gunbuster and Diebuster (again claimed to go below Absolute Zero, to some ridiculous number of negative trillions of degrees)
- Captain Cold from DC Comics with his freeze ray
- Cygnus Hyoga from Saint Seiya via his Aurora Execution attack
- Evangeline A.K. McDowell from Negima! via her Kosmikē Katastrophē spell
- Iceman from Marvel Comics with his mutant powers
- Kal Su from Bastard!! via his Testament spell
- Kiryu (MechaGodzilla III) from the Godzilla movies.
- Liber Legis from Demonbane via Hyperborea Zero Drive (despite being Absolute Zero, it can destroy matter beyond subatomic and far below moleculer level, froze soul and spillover can trigger dimensional quake, causing distortion of time/space and a lot of other things)
- MetalGarurumon from Digimon via his Cocytus Breath attack. Omnimon can also fire an absolute zero blast from his Supreme Canon
- Red Mage of Eight Bit Theater's Ice 9 spell
- Tokiya Mikagami from Flame of Recca
- White Album from Jojo's Bizarre Adventure