A quid is a pound sterling, in British slang. Some residents of British dependencies also use this term, and in Ireland, people may refer to a euro as a quid as well, just to add to the confusion. Britain has been extremely resistant to the adoption of the euro, despite pressure from other members of the European Union, and the British pound is likely to remain a viable unit of currency for some time.
The origins of the term "quid" are a bit difficult to nail down. The term has been used since the late 1600s to describe the pound, and it may be derived from the Latin quid pro quo, or "something for something," implying some sort of exchange. It may also be a reference to the former site of a Royal Mint papermill in Quidhampton, Wiltshire. Whatever the origins of the word, it is in wide use in England as a familiar slang term for a pound, and prices are sometimes denoted in quid, especially in casual advertisements.
Formally, the British pound is known as the pound sterling, or simply "pound." The use of the pound dates back to Anglo-Saxon times in England, although it has undergone a number of changes since then. Pounds are denoted with the symbol £, which is always placed before the number without a space, as in £40,000. In 1971, Britain abandoned the complex fractional system of shillings and pence for a decimal system, denoting one pound as 100 pence, much to the delight of frustrated British math students.
The long history of the British pound has been a big part of the reason why the British are reluctant to abandon the quid for the euro. Several nations in the European Union have struggled with currency conversion, because conversion to a universal currency, while having obvious advantages, can also feel like a loss of an important part of history. The British government has said that it will not convert to the euro unless it can determine that the conversion is in the "national interest," in the words of Tony Blair, a former Prime Minister.
The plural of quid is also "quid," and the word is usually used after the quoted denomination, as in "nine quid," rather than "quid nine." This piece of British slang often crops up in fiction and film set in Britain, especially when the characters come from the lower classes, and usage of the term varies in different parts of Britain and her dependences. As a general rule, guests stick to "pound," rather than "quid," unless they want to sound affected.