Decapitation is when somebody's head gets chopped off from their body.
Imagine you have a toy doll with a head that you can take off. That's what happens during decapitation, but instead of a toy doll, it is a real human body.
Normally, our heads stay on our bodies because there is a big bone called the spine that connects the head to the rest of the body. But if somebody chops off the head with a sharp object, like a sword or an axe, then the spine breaks and the head comes off!
Decapitation is a very serious thing, and it can be very dangerous or even fatal. It is not something you should ever do or let happen to you or anyone else.