1. (In the US) 0.473 liters (16 fluid ounces) of a liquid, typically draft beer.
2. (In the UK) 0.568 litres (20 fluid ounces) of a liquid, typically draught beer.
3. (Metric - everywhere else in the world other than UK/US) Exactly half a litre (or 500mL, 2 cups, 18 fluid ounces, etc.) of any liquid, most notably beer.
Man (walks into a bar in Toronto): I'll have a pint of Guiness.
Bartender: One pint of Guiness, coming up.
Man: So a pint is 375 mL, right?
Bartender: No sir, its 500mL.
Man: Well I'll be damned. But it's 12 ounces right?
Bartender: No sir, its 18 fluid ounces.
Man: Damn, I'll have to stop getting my facts from urbandictionary.com
Bartender: Well actually, I'm a psychic, and I'm having a vision of some valiant soul deciding to post a proper definition of the word "pint" as it refers to drinking. So don't worry, keep on urbandicking!
Man: Urbandicking? I'll have to look that one up.
Bartender: Well if its not self-explanatory then you're an idiot.
Man: Just give me the goddamn pint.