Nirvikalpa Samādhi

Nirvikalpa is a Sanskrit adjective with the general sense of "not admitting an alternative", formed by applying the contra-existential prepositional prefix निः ni ("away, without, not") to the term विकल्प vikalpa ("alternative, variant thought or conception").

In Raja Yoga, nirvikalpa samadhi is a synonym for Asamprajnata Samadhi, the highest stage of samadhi.Heinrich Zimmer in his book distinguishes Nirvikalpa Samadhi from other states as follows:

Nirvikalpa samādhi, on the other hand, absorption without self-consciousness, is a mergence of the mental activity (cittavṛtti) in the Self, to such a degree, or in such a way, that the distinction (vikalpa) of knower, act of knowing, and object known becomes dissolved — as waves vanish in water, and as foam vanishes into the sea.

According to Swami Sivananda, it is also called Nirbija Samadhi:

"Without seeds or Samskaras [...] All the seeds or impressions are burnt by the fire of knowledge [...] all the Samskaras and Vasanas which bring on rebirths are totally freed up. All Vrittis or mental modifications that arise from the mind-lake come under restraint. The five afflictions, viz., Avidya (ignorance), Asmita (egoism), Raga-dvesha (love and hatred) and Abhinivesha (clinging to life) are destroyed and the bonds of Karma are annihilated [...] It gives Moksha (deliverance from the wheel of births and deaths). With the advent of the knowledge of the Self, ignorance vanishes. With the disappearance of the root-cause, viz., ignorance, egoism, etc., also disappear."

Nirvikalpaka yoga is a technical term in the philosophical system of 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.

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Wikipedia