Nirvikalpa is a Sanskrit adjective with the general sense of "not admitting an alternative", formed by applying the contra-existential prepositional prefix ("away, without, not") to the term ("alternative, variant thought or conception").
, on the other hand, absorption without self-consciousness, is a mergence of the mental activity () in the Self, to such a degree, or in such a way, that the distinction () of knower, act of knowing, and object known becomes dissolved — as waves vanish in water, and as foam vanishes into the sea. The difference to the other samadhis is that there is no return from this samadhi into lower states of consciousness. Therefore this is the only true final Enlightenment.
Nirvikalpaka yoga is a technical term in the philosophical system of Kashmir Shaivism, in which there is a complete identification of the "I" and Shiva, in which the very concepts of name and form disappear and Shiva alone is experienced as the real Self. In that system, this experience occurs when there is complete cessation of all thought-constructs.
In Buddhist philosophy, the technical term is translated by Edward Conze as "undifferentiated cognition". Conze notes that only the actual experience of can prove the reports given of it in scriptures. He describes the term as used in Buddhist context as follows:
The "undiscriminate cognition" knows first the unreality of all objects, then realizes that without them also the knowledge itself falls to the ground, and finally directly intuits the supreme reality. Great efforts are made to maintain the paradoxical nature of this gnosis. Though without concepts, judgements and discrimination, it is nevertheless not just mere thoughtlessness. It is neither a cognition nor a non-cognition; its basis is neither thought nor non-thought.... There is here no duality of subject and object. The cognition is not different from that which is cognized, but completely identical with it.