What is co - efficient of restitution

"COR" stands for "coefficient of restitution" which is a measurement of the loss or retention of energy when two objects collide. When the club head hits the ball at impact, the energy from the club head that is swinging at a certain speed is transferred to the golf ball which then gets airborne and travels towards the target.

The COR measurement is always expressed as a number between 0.00 and 1.00. A COR of 0.00 means that all the energy is lost in the collision at impact whereas a COR of 1.00 means a perfect collision where all energy is transferred from the club head to the golf ball without the loss of any energy – an ideal case situation for a golfer
In reality, it is impossible for the collision of the golf club and golf ball to produce a perfect collision (COR of 1.00) in which all energy is transferred, for the reason that the clubface and the ball are made from completely different materials as well as the club head and the ball are of two totally different weights, or masses.
The current USGA rule limiting the coefficient of restitution of a club head states that the COR cannot be higher than a measurement of 0.83. This means that when the club head strikes the ball, there cannot be more than an 83-percent transfer of the energy of the club head to the ball.
As an example, if a player swinging at 100 mph swing speed plays with two drivers – the first with a COR of 0.82 and the second with a COR of 0.83, he would hit the second driver 5 yards longer than the first driver. It is also true that as swing speed increases, the distance difference is greater. And likewise, as swing speed decreases the distance difference for each increment of the COR measurement is less.