trying to define it is hard, but heres me veiw:

the punch needs to be:

unnoticed by your opponent: if they dont see the punch coming they dont have time to prepare. this means a number of things, in that their chin will be in the wrong position (generally)

When the punch collides with your opponents head, the direction the head moves in comparison to the body and neck.
you ideally want to be snapping their head from side to side, as this will temporarily cut off their nerve endings, aswell as some of the oxygen supply to their brain, causing them to basically pass out.
The nerve ending sometimes add hilarity, look at the zab judah dance...

the speed and power of the punch are the factors which control the 2 above mentioned.
speed allows unnoticed/unprepared for punches, and power adds to the sudden jerking of the head.

its also important to follow through your punch as it hits its target, aimint to punch to the back of the head, but once you have connected you need to get that hand out of there.
not only for defensive reasons, but leaving your punch out disperses the enegry of the punch, reducing the force. youll end up with more of a push than a punch