Male copulative organ. Male reproductive organ containing the urethra, the channel through which urine and semen are voided. It transfers sperm to the female reproductive tract to fertilize the ovum. In mammals, the penis is made erect by vessels that fill with blood, and in most mammals (but not humans) is stiffened by a bone.
Snakes and lizards have a paired structure that serves as a penis; other reptiles have a single organ. A few birds, mainly ducks and geese, also have a type of penis, as do snails, barnacles, and some other invertebrates. Many insects have a rigid, nonerectile male organ, usually referred to as an intromittent organ.
A 1995 us study showed the average length of an erect human penis to be 12.8 cm/5 in (smaller than is popularly assumed).
The male organ of copulation ('member' is a euphemism); SYN. phallus, member.