A decapitated Beta-Hirogen
Badgey manually decapitating a holographic Bajoran
Decapitation or beheading, was a form of dismemberment involving the removal of a humanoid's head. It could be the result of combat, an accident, or a form of execution. A bat'leth was capable of performing a decapitation with a single stroke. (ENT: "Marauders")
During the Andorian incursion at the monastery at P'Jem, the Andorian Tholos threatened to decapitate a few Vulcan monks in order to pressure Captain Jonathan Archer into telling the truth regarding his presence there. (ENT: "The Andorian Incident")
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 this statement by informing Kirk that the phrase was attributed to a prince who was betrayed and decapitated by his own subjects. (Star Trek Into Darkness)
In 2266, concerned about James T. Kirk's adoption of Thomas Leighton's suspicion that Anton Karidian was really Kodos the Executioner, Leonard McCoy asked him what he would do if it was true: "Do you play God, carry his head through the corridors in triumph?" (TOS: "The Conscience of the King")
Decapitation could also be used in a metaphorical sense to describe the death of something intangible. In 2267, McCoy criticized Spock for playing chess with the starship's computer, saying that Captain Kirk's professional head was about to be lopped off. (TOS: "Court Martial")
In 2268, a captured Arne Darvin wished for his statue in the Klingon Hall of Warriors to depict him standing with Kirk's head in one hand and a tribble in the other, following the successful completion of his plot to kill Kirk. (DS9: "Trials and Tribble-ations")
While revisiting the Shore Leave Planet in 2269, the Queen of Hearts threatened Leonard McCoy with decapitation by yelling "Off with his head!". (TAS: "Once Upon a Planet")
During the Battle of Ajilon Prime, two doctors took time to discuss 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, an individual's headless corpse could be the last thing he or she saw. (DS9: "Nor the Battle to the Strong")
Bat'leth attacks were known for their potential to kill with one clean stroke. A Tellarite serving under Captain Laporin died in such a manner. (ENT: "Marauders"; DS9: "Nor the Battle to the Strong", "Apocalypse Rising")
Sural, the dictator of Sakura Prime, was known for having beheaded both his rivals and his parents. (VOY: "Dreadnought")
In 2374, Tom Paris found the decapitated head of a Beta-Hirogen aboard a Hirogen ship, still wearing his helmet. (VOY: "Prey")
In 2380, while the holodeck of the USS Cerritos was malfunctioning, Badgey's program became corrupted, and he became murderous. He went so far as to rip the head off a holographic Bajoran while he was pursuing Ensigns Sam Rutherford and D'Vana Tendi. (LD: "Terminal Provocations")
In 2399, Elnor confronted Tenqem Adrev as he attacked Jean-Luc Picard on Vashti. After Adrev disregarded Elnor's warning, Elnor decapitated him with a single stroke of his tan qalanq. (PIC: "Absolute Candor")
In 2401, Sneed had Lurak T'Luco decapitated, and was later decapitated himself by a kur'leth when Worf was rescuing Raffaela Musiker from Sneed and his henchmen. (PIC: "Disengage")
Sometime prior to 3189, Zareh assumed the role of courier for "the Colony" by beheading the planet's previous courier. (DIS: "Far From Home")