A unit of volume, equivalent to:
(British, metonymic) A pint of milk.
Example: Please leave three pints tomorrow, milkman.
(UK, metonymic) A glass of beer or cider, served by the pint.
Example: A couple of pints please, barman.
But if you're 16, so two years under the legal drinking age, if you have a carvery, they are allowed to serve you a pint of beer.