From Damn Interesting:
For thousands of years, the forceful removal of the human head has been used as a form of capital punishment. In fact, the word “capital” in the context of punishment was coined to describe execution by decapitation, derived from the Latin word caput, which means “head.” Since the very beginnings of the practice, there has been much speculation and debate regarding the length of time that the head can remain conscious after its removal. Many argue that a beheaded person will almost instantly lose consciousness due to a massive drop in blood pressure in the brain, and/or the heavy impact of the decapitation device. But there are countless eyewitness reports in history describing a few moments of apparent awareness in the victim.