The Definition of Karma

 A nondual friend of mine doesn't believe in karma, and whenever I mention it, he looks at me like I am caught in a silly superstition.

I think he doesn't define karma the way I do. Probably he thinks of it the way most Westerners were taught: If we do good, we reap good karma in our next lives. If we do bad things, we likewise reap that. So a human being may be born to a higher status or perhaps as an angelic being if he does good, and as a dog or worse if he does bad. This definition requires belief in both reincarnation and moral behavior as the most important virtue. 

My definition is quite different. I'm not sure if it is the standard view among nondualists or if my view is just what makes sense to me. Karma is the result of thoughts believed in. As long as we believe our thoughts (including unconscious ones), our actions are governed by them and we (and others) reap the results. 

After awakening, thoughts are seen to be empty -- having no substance. We may go on believing some of the most persistent ones for a long time, so it's not like the slate is necessarily wiped clean, but the heaviness of karma is nonetheless lifted once we know our beliefs are only just that -- beliefs.