Erection

An erection (clinically: penile erection or penile tumescence; informal/slang: stiffy, hard-on, boner and many others) is a physiological phenomenon of the male genitalia of many species, 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 usually, though not exclusively, associated with sexual arousal or sexual attraction. The shape, angle and direction of an erection varies considerably in humans.