Sri Sri Ravi Shankar:
Nirvana is bringing the balance and having no feverishness for any desire. Desire means a lack. When you say, I want nothing, I am content, that is nirvana. But that doesn't mean you shun your duties. You perform your duties well but you are with your center. Even craving for enlightenment is an hindrance to enlightenment.
All emotions are linked with people, objects and events. Catching on to objects, people or relationships hinders freedom, liberation. When the mind is free from all impressions and concepts, you are liberated. The state of nothingness is called Nirvana, enlightenment, Samadhi. Nirvana is getting back to me from mine. Who am I? When you go deep inside you, layer after layer, that is nirvana. It is like peeling an onion! What do you find in the center of an onion? Nothing!
When you know that everything is changing - all relationships, people, body, feelings - suddenly the mind which clings on to misery comes back to you. The return from mine to me, gives you contentment and freedom from misery. Reposing in that state of contentment is nirvana.