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
, such as at REM sleep (colloquially known as ''morning wood'' (AE) or ''morning glory'' (BE) when present when a man wakes up).
== See also ==