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1.
A rather scientific sounding word (especially when used when casually reffering to sex), when really it's all quite simple. One thing 'enters into' another thing.
All we really want to know is: was there any penetration?
by Jacs March 01, 2005
 
2.
Penetration is the medical term for the penis entering into the vagina. It's a good way term for talking about sex if one wants to seem mature. Even if you're a dirty little pervert, you'll sound like sex is more than just an emotionless action caused by hormones if you call it 'penetration' rather than 'fucking around,' 'doing it,' etc.
Dude 1: Did you fucked your girlfriend last night?

Dude 2: Penetration occured and I depostited my semen inside of her vagina.

Dude 1: What the fuck? You're a frat boy, you don't talk like that.

Dude 2: I know, but I sure sound mature, don't I?
by Tit For Twat January 26, 2007
 
3.
In sex the disappearence of the penis into the vaginal cavity and the rupturing the hymen.
Kiersten's examination in the ER showed that the source of her vaginal bleeding was penetration and the puncturing of the back wall of her vagina as the source of the blood. Kiersten spoke of falling on her bicycle handlebar, but the experienced personnel in the ER knew rape when they saw it. Who was the rider of the second bike that was found? Someone said Kiersten had been seen riding with Peter Goesinya.
by Richard Black March 25, 2005
 
4.
to "poke" someone in a bodily orifice
COME OVER HERE AND PENETRATE ME BIG BOy
by bernard February 02, 2005