My definition of bad karma is when you don't do something right by someone and it comes back to bite you in the a$$ somehow. Like if you were to steal someone's wallet, then you get your own wallet stolen the same day.
My definition of good karma is kind of like "pay it forward." Such as if I bought a cup of coffee for someone, and then someone else buys me a cup of coffee later that day (and especially without even asking.) Some could interpret that as just simple kindness, and not karma, though.
