Nirvana simply means cessation. It is the cessation of passion, aggression and ignorance; the cessation of the struggle to prove our existence to the world, to survive. We don't have to struggle to survive after all. We have already survived. We survive now; the struggle was just an extra complication that we added to our lives because we had lost our confidence in the way things are. We no longer need to manipulate things as they are into things as we would like them to be. -- Introduction to Buddhism
Nirvana is ineffable. It cannot be described. Literally it means "blowing out," as if referring to a lamp. In nirvana, all "individual personality or ego ceases to exist and there is nothing to be reborn." (The Buddhist Tradition, 12)
The conception of Nirvana differs in the Mahayana and Theravada sects of Buddhism.
Hinduism calls this idea Moksha.
In Zen, the absolute refers to the essential Oneness of everything. It is often known as nirvana or the undifferentiated state, as opposed to Samsara, the differentiated state.