Simple Definition:

Karma is the idea that every action has consequences, either in this life or in future lives, depending on whether it is good or bad. [Consequences means the results or effects of an action.] For example, if someone helps others, they will experience good karma and may have positive things happen to them in the future. On the other hand, if someone harms others intentionally, they will experience bad karma and may have negative things happen to them in the future.

Very Simple Definition:

Karma means that what goes around comes around. If you do good things, good things will happen to you, and if you do bad things, bad things will happen to you. For example, if you help someone, someone may help you back in the future. If you hurt someone, you may get hurt in the future.