One unit of the UK pound sterling (GBP).
For most of the historical period up the early 19th century, it could - in theory - be exchanged for one pound (weight) of silver.
Thereafter the pound was generally backed by gold (the 'gold standard').
Since 1931, the pound sterling has been a fiat currency.
A pound is also one unit of a number of other currencies including those of Egypt (EGP), Lebanon (LBP), South Sudan (SSP), Sudan (SDG) and Syria (SYP).