From Webster's: Karma: often capitalized : the force generated by a person's actions held in Hinduism and Buddhism to perpetuate transmigration and in its ethical consequences to determine the nature of the person's next existence.
The "official" definiton refers to the conseqences of one's actions materializing in the next life.
In everyday usage though it seems to mean "what goes around, comes around." In other words, if you're nice the universe will reward you, if you're mean life will punish you.
Do you believe that? Is justice manifested in this life or the next?