Cable (or the cable) is a foreign exchange term used for the GBP/USD currency pair rate (British pound vs the US dollar).
The term cable is a slang term used by forex traders to refer to the exchange rate between the pound and dollar and is also used to simply refer to the British pound itself. The term originated in the mid-19th century, when the exchange rate between the U.S. dollar and British pound began to be transmitted across the Atlantic by a submarine communications cable. Since that time the exchange rate has been referred to as the cable.
The first Transatlantic Cable was laid under the Atlantic Ocean in 1858, but it failed after only about a month of fitful service. The first truly successful cable across the Atlantic was completed in July 1866, reliably transmitting currency prices between the London and New York City Exchanges. The first such exchange rate to be published in The Times appeared in their issue of 10 August 1866.
Transatlantic communications are now mainly carried by optical fibre cables, supplemented to a small degree by satellites, but forex traders' nickname for the pound-dollar pair still hearkens back to the old days of copper telegraph cables.