Simply, a shot
from a firearm
to the head area
of a human
or other animal target
Generally, a shot that sufficiently penetrates the cranial cavity will cause immediate incapacitation and mortality.
Though quite difficult to achieve in a high tension scenario such as a self defense situation, a head shot is the only sure way to stop an attack immediately. The human body is just too elastic and strong to be immediately incapacitated with even the most powerful handgun rounds.
This does not mean that one should always aim for the head especially in a self defense scenario as the risk of missing is high. It is always better to try and put two shots into the target's center of mass first and if warranted one to the head to quickly end the encounter in your favor.
Police officers and military personnel are trained to aim for center of mass when engaging hostile targets.
It is possible though that some hostage rescue groups are trained to take head shots first as hostage situations are even much more tense and the need for immediate incapacitation of multiple and possibly armored targets is more real and dire.