The unitive experience stands among the most contested and generative concepts traversing depth psychology, mystical philosophy, and transpersonal research. The corpus registers it across at least four distinct registers: Neoplatonic metaphysics, psychedelic phenomenology, Zen-psychoanalytic dialogue, and contemplative-yogic tradition. Plotinus provides the oldest and most philosophically rigorous account, describing a state in which knower and known, seer and seen, become fused — an event that cannot be willed but arrives by grace, straining language to its limits. Grof’s LSD research translates this ancient category into clinical phenomenology, documenting ‘cosmic unity’ and ‘oceanic ecstasy’ as recurring transpersonal phenomena whose structural signature — transcendence of subject-object dichotomy, dissolution of time and space, blissful affect, revelatory insight — maps strikingly onto cross-cultural mystical reports. Griffiths and colleagues operationalize these features further in psilocybin research, disaggregating ‘internal unity,’ ‘external unity,’ and ‘transcendence of time and space’ as measurable subscales. Cooper’s Zen-psychoanalytic work introduces a non-dualistic inflection, arguing that the unitive nature of reality is not a peak state to be attained but an ongoing ground to be realized and enacted. Johnson and Welwood treat it as the visionary revelation of underlying wholeness that active imagination and nondual awareness respectively make accessible. Across these positions, a central tension persists: whether the unitive experience dissolves selfhood permanently, transfigures it, or simply discloses a depth always present beneath ordinary consciousness.