It is the cessation of suffering, the liberation from karma, and therefore the passing over into another existence. The best way to think about nirvana is that it is the final goal of Buddhism, and that Enlightenment is the step immediately before it. Thus one becomes aware of the nature of Ultimate Reality in Enlightenment, and then one becomes unified with that reality in nirvana. Thus the Buddha, when he died, passed into Nirvana, having previously attained Enlightenment during his life and sharing it with humanity. A bodhisattva is one who has attained Enlightenment, but rather than passing over into nirvana, chose to come back to this world to use their power to help other people.