An
erection (clinically:
penile erection or
penile tumescence) is a physiological phenomenon in which the
penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is often associated with
sexual arousal or
sexual attraction, although erections can also be spontaneous. The shape, angle and direction of an erection varies considerably in humans.