A punch to the back of the head or base of the skull from behind. It is an illegal punch in boxing, due to the danger it poses to the receiving boxer. It can cause severe injury, and even death.
Why are rabbit punches so dangerous?

The area at the base of the skull is relatively unprotected, and near a point at which the brain meets the spine, connected by the brainstem. Due to the vulnerability of this area and severity of the potential damage that can be caused, rabbit punches are illegal and referees are strict about preventing them. Boxers are warned if these punches are thrown, points can be taken away, and boxers can be disqualified.
Are rabbit punches deliberate?
In the normal course of boxing, both fighters are moving around and sometimes these punches can be landed accidentally. Most commonly if a boxer turns his back on an opponent who is already throwing, or if they bend over. Sometimes a punch may hook around the back unintentionally, especially during a clinch.
There are also times when deliberate fouling occurs, and a boxer may use angles to inflict a foul that can be less easily seen by the referee. This can include punches to the back of the head.
There are calls for more action to be taken against rabbit punching in boxing due to its risk.
Why is it called a rabbit punch?
The name is taken from hunting, where a caught rabbit could be killed by a blow to the back of its head.