The erection of the penis, clitoris or a nipple is its enlarged and firm state. It depends on a complex interaction of psychological, neural, vascular and endocrine factors. The ability to maintain the erectile state is key to the reproductive system and many forms of life could not reproduce in a natural way without this ability.
Erections often result from sexual arousal, though there can be other causes too.
 Penile erection
In males, sexual arousal typically shows in a penile erection, which is a swelling, hardening and enlargement of the penis. Such an erection enables sexual intercourse and is also usually present during masturbation. It can sometimes happen that a teenager or man being spanked in the over the knee position gets an erection. The erection can then rub against the spanker's lap, ultimately leading to orgasm and ejaculation. After ejaculation, the erection usually ends.
A non-sexual cause of penile erections is REM sleep. The phenomenon is colloquially known as morning wood (AE) or morning glory (BE) when present when a man wakes up.
An inability to develop and/or maintain a penile erection is called erectile dysfunction or "(male) impotence". It can occur due to both physiological and psychological reasons, most of which are amenable to treatment.
 Clitoral erection
Clitoral erection is a part of sexual arousal in women. The clitoris is the anatomically homologous counterpart of the penis, and the physiological mechanism of its erection is similar. Swelling and enlargement may also occur during a clitoral erection but because a large proportion of the clitoris is within the body and its drastically reduced size, it is often not as obvious.
 Nipple erection
Nipple erection may result from three kinds of response. It happens in females during breast feeding. It is also an early part of the sexual response in females and males. Both of these are caused by the release of oxytocin. Nipple erection can also be caused by a tactile response to cold temperature in both males and females. The erection of nipples is not due to erectile tissue, but due to the contraction of smooth muscle under the control of the autonomic nervous system. It is more akin to a hair follicle standing on end than to a sexual erection.
 See also
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