Decapitation or beheading is the total separation of the head from the body. Such an injury is invariably fatal to humans and most other animals, since it deprives the brain of oxygenated blood, while all other organs are deprived of the involuntary functions that are needed for the body to function.

The term beheading refers to the act of deliberately decapitating a person, either as a means of murder or as an execution; it may be performed with an axe, sword, knife, machete or by mechanical means such as a guillotine or chainsaw. An executioner who carries out executions by beheading is sometimes called a headsman. Accidental decapitation can be the result of an explosion, a car or industrial accident, improperly administered execution by hanging or other violent injury. Suicide by decapitation is rare but not unknown. The national laws of Saudi Arabia, Yemen, and Qatar permit beheading; however, in practice, Saudi Arabia is the only country that continues to behead its offenders regularly as a punishment for capital crimes.

Less commonly, decapitation can also refer to the removal of the head from a body that is already dead. This might be done to take the head as a trophy, for public display, to make the deceased more difficult to identify, for cryonics, or for other, more esoteric reasons.