Decapitation was the removal of a humanoid's head. It might be the result of combat, or a form of execution.
In 2259 of the alternate reality, when James T. Kirk tried to justify his decision to "use" Khan Noonien Singh with the Arabic proverb "The enemy of my enemy is my friend" Spock countered by informing him that the phrase was attributed to a prince who was betrayed and decapitated by his own subjects. (Star Trek Into Darkness)
Decapitation could also be used in a metaphorical sense to describe the death of something intangible. In 2267, Leonard McCoy criticized Spock for playing chess with the ship's computer, saying that CaptainKirk's professional head was about to be lopped off. (TOS: "Court Martial")
During the battle of Ajilon Prime Ajilon Prime two doctors took to discussing different ways of dying. According to one doctor, a nice clean decapitating blow had its virtues. Another doctor countered that the brain lived on for five, ten seconds at least, and that in theory, your headless corpse could be the last thing you saw. (DS9: "Nor the Battle to the Strong")
bat'leth attacks were known for their potential to kill with one clean stroke. (ENT: "Marauders"; DS9: "Nor the Battle to the Strong", "Apocalypse Rising")
During the Andorian incursion at P'Jem, the Andorian soldier Tholos threatened to decapitate a few Vulcan monks in order to pressure Jonathan Archer the thruth of his being there. (ENT: "The Andorian Incident")
While visiting the Shore Leave Planet for the second time in 2269, the Queen of Hearts threatened Leonard McCoy with decapitation by yelling "Off with his head!". (TAS: "Once Upon a Planet")
The dictator of Sakura Prime was known for having beheaded both his rivals and his parents. (VOY: "Dreadnought")