Sakshi, Sanskrit for witness, names the pure awareness that observes the mind's activity, the body's sensations and the world's changing contents without itself participating in or being altered by any of it, a concept central to both Advaita Vedanta and the dualist Samkhya philosophy that influenced classical Yoga. In Samkhya, the witness is purusha, pure consciousness, categorically distinct from prakriti, the entirety of active, changing nature including the mind and its thoughts; liberation on this account is the witness recognizing itself as forever separate from the activity it merely observes.
Advaita Vedanta uses sakshi in a structurally similar way as a teaching device, pointing students toward the observing awareness present in every state, waking, dreaming and sleeping, as a first step toward recognizing turiya, though Advaita ultimately treats even the distinction between witness and witnessed as provisional, a useful stage on the way to the deeper non dual claim that witnessing awareness and its apparent objects are, at the most fundamental level, not actually two separate things. The term appears throughout classical yoga and Vedanta commentary as a technical anchor point, distinguishing the unchanging observer from the observed contents of mind that Samkhya groups together under the single heading of prakriti.