Erection

An erection, clinically: penile erection or penile tumescence is a physiological phenomenon in which the penis becomes firm, 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 between humans. The state of a penis which is partly, but not fully, erect is sometimes known as semi-erection; a penis which is not erect is typically referred to as being flaccid, or soft.