Firstly the technical stuff, viz.

A knockout (also referred to as a K.O.) is a fight-ending, winning criterion in boxing.  A full knockout is considered any legal strike or combination thereof that renders an opponent unable to continue fighting.  The term is often associated with a sudden traumatic loss of consciousness caused by a physical blow.  Single powerful blows to the head (particularly the jawline and temple) can produce a cerebral concussion or a Carotid sinus reflex with Syncope and cause a sudden, dramatic KO.  Body blows, particularly the liver punch, can cause progressive, debilitating pain that can also result in a KO.

In boxing a knockout is usually awarded when one participant falls to the canvas and is unable to rise to his feet within a specified period of time, typically because of exhaustion, pain, disorientation, or unconsciousness.  For example, if a boxer is knocked down and is unable to continue the fight within a ten second count, he is counted as having been knocked out and his opponent is awarded the K.O. victory.

What I love about the K.O. is that it renders any subjectivity regarding the bout redundant....There is no discussion....period.  Normally throughout a bout you have an inkling as to the outcome, however, any boxer that has a knockout punch in his arsenal is capable of winning right up to the bell signaling the end of the bout.

Please enjoy the following footage; there is a mixture of devastating knockouts coupled with historical ones too.