One pound (100 pence) in British Sterling (GBP). Can refer to the actual denomination (a goldish looking coin, about a centimetre and a half across), or any amount in Sterling. In very common use, all across the UK.
"That'll be a quid mate"
"That's going to set you back about 3, 4 hundred quid"
A singular unit of currency, once recognized only as the British pound (100 pence) but now widely used in place of the word dollar, pound, peso, et cetera.
Man: I'll give you twenty quid.
Whore: Sorry, mate, that's not enough to get in my boot!