In psychoanalysis, the particular internal origin of an instinct (3), being either the site of its origin, such as an erotogenic zone, including a sexual organ, or the physiological process assumed to occur at such a site and to be experienced as excitation. Sigmund Freud (1856–1939) introduced the concept in 1905 in his book Three Essays on the Theory of Sexuality and developed it fully in 1915 in an article on ‘Instincts and their Vicissitudes’ (Standard Edition, XIV, pp. 117–40, at pp. 123, 132). Compare instinctual aim, instinctual object.