Karma simply means action. More specifically, the idea of cause and effect. Do something and it will yield effects/results. This is pretty much common sense. We live in a very interconnected world, so almost anything we do will effect someone or something else. Many times, the result is unseen or unintentional.
I accept karma in this life as a fact of basic physics and the result of living in a causal universe.
The idea of karma as related to other lives or incarnations is an entirely different matter, however. As I do not believe in reincarnation or any sort of afterlife, I assert that any notion of karma is limited to this life.