What is an Erection?

Definition: An erection is a state when two tubular structures in the penis become engorged with rapid flowing blood. Since more blood is entering than exiting the penis, it has to make more room for the blood so it stretches out, making the penis longer and stiffer. The shape will vary - some men have an erection that goes straight out or up, and some have one that curves a little bit. If yours looks odd to you, there are books and websites that show you photos of normal erections, but individuals vary somewhat in their appearance.

An erection is a normal part of the male sexual response, although it can be a bit embarrassing when it occurs in a public spot. All men get erections, even as babies, though erections in young boys and babies are usually a response to physical touch. You will start getting your first sexual erection when you hit puberty, which can be anywhere from 9 or 10 to 14 or 15; during that time you will have what are called "wet dreams" where your body ejaculates during your sleep. This is also totally normal!
Slang terms for 'erection' include: Boner, Stiffy, Hard-On, Woody

What Causes Erections?
Sexual excitement causes most erections. Men are visual animals, and things they see - like a good-looking person, someone with a skimpy outfit on that shows a lot of skin, a pornographic picture, or just about anything remotely sexual - will cause an erection. Things they are thinking about, and things that are touching the penis will also cause an erection. When you first hit puberty, though, you can get an erection at any time, without really thinking about anything sexual. This can be really embarrassing for teenage boys, but try to remember that it's a normal part of growing up. Your body is just starting to make hormones, and sometimes they get out of hand and cause unwanted responses.
Male erections can occur at any time of day or night. Men get erections when they are sexually aroused and sometimes randomly.
Erections can be triggered by gentle rubbing of the penis, especially with something smooth and silky.
They often occur in bed and when asleep.
Warm water in a shower is also very likely to trigger one.
Teenage boys probably get them most as they haven't learned to control them by their thought processes. Older men can do this to some extent.
An erection happens during stimulation. the parasympathetic nervous system is responsible for increased blood going to the penis causing it to become erect. Normally this is a preparation for sexual activity.
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NOTE:

The word 'erection' is not solely related to a turgid penis - it can also refer to a construction, such as a building or some other upright structure.