Samadhi, from Sanskrit roots meaning to bring together or to concentrate fully, names the eighth and final limb of the eightfold path Patanjali sets out in the Yoga Sutras, the culmination of a sequence that runs from ethical restraint through posture and breath control to progressively deeper stages of concentration, dharana, and meditation, dhyana. In Patanjali's system, samadhi is reached when meditative absorption becomes so complete that the meditator's sense of being a separate observer dissolves into the object of meditation itself, awareness and its object no longer experienced as distinct.
The term is not exclusive to classical Yoga: Hindu traditions generally, Jain meditative practice and the whole of the Buddhist meditative path use samadhi or close cognates for comparably deep states of unified concentration, though each tradition situates the state within its own account of what concentration is ultimately for, liberation from rebirth in most Indian frameworks, but reached through different metaphysical routes and, in Buddhist usage particularly, without the assumption of a permanent self that non-Buddhist Indian traditions typically retain. Patanjali describes the final three limbs together, concentration, meditation and samadhi, as samyama, a combined discipline whose mastery is said to yield the extraordinary powers, siddhis, discussed later in the Yoga Sutras.