What is the difference between a quid and a pound?

The words "quid" and "pound" are interchangeable terms used to refer to one pound sterling in British currency. Quid is an informal colloquial slang while pound is a more formal shortening.

Though the use of the word quid to describe a pound can be traced as far back as the 17th century, its actual origins are obscure. The most likely source is as a derivation of the Latin word "quid," meaning "what, something or anything". Another possibility is that use of the word came about as a reference to the Royal Mint paper mill located in Quidhampton which once produced paper for banknotes