Resurrection is the ability to revive oneself or others after they have died. Often, characters can do one or the other, not both, but others can perform both. The limits of this ability vary from fiction to fiction and from character to character; some may be only able to resurrect those who have recently died, while others may be capable of bringing those who have been dead for quite some time back to life, even if nothing remains. The mechanisms vary as well.
When in the hands of a character who uses resurrection on themselves, giving them some degree of Immortality, this ability often co-exists with Regeneration, though the process can take some time. This ability is not to be confused with Necromancy, the ability to raise the dead as undead beings, but it also covers Reincarnation, characters who, upon death, eventually reincarnate the bodies of others - not as a mechanic of the verse, but through their own powers.
In many cases, self-resurrection may take too long to be combat applicable, but it still exists. The ability has similar limitations to regeneration, and one should not assume that, because a character has never failed to resurrect, that they can resurrect regardless of how they've been killed.