A basic natural law, generally completely misunderstood in the West. It has nothing to do with morality, with good and evil, or with the so called karmic credits. It has nothing to do with justice, and certainly not with any kind of judgement. It is not about reward or punishment. It also has nothing to do with the belief in reincarnation - it just happened to be first discovered by people who also believed in reincarnation. And, despite Webster's definition, it has nothing to do with fate.
It is a completely impersonal law of nature, understood for centuries by the Hindu and Buddhists, and partly rediscovered by Sir Isaac Newton.
The word karma means action in Sanskrit. The law of karma is almost identical to Newton's law of action and reaction. Essentially, Newton's law is a special case of the law of karma. The law of karma is:
"For every action there is an equal reaction in the opposite direction when an occasion arises."
Save for the "when an occasion arises" it is identical to Newton's law of action and reaction. The simple reason Newton did not add the occasion clause was the fact he based his law on the observation of strictly physical, even mechanical phenomena. It so happens that on the physical/mechanical level the occasion always arises at the moment of the action. Hence, the immediate reaction.
Had Newton based his law on the observation of chemical, biological, let alone psychological and sociological phenomena, he would have re-discovered the law of karma in its fullest meaning. He would have still called it the law of action and reaction, and he would have been perfectly correct. After all, karma and action are exactly the same, except one is Sanskrit, the other English (or actually, Latin). | 
