The Piece of Eight is a silver coin minted by the King of Spain in the Spanish Americas from the end of the 15th century and used as currency in the Spanish Empire. It was valued at eight reales, the unit of Spanish currency, and could be cut into halves, quarters or eigths to make change, hence 'pieces of eight'. In Spanish, the coin was called 'Peso de Ocho'. The silver for the coin was mined mostly in Bolivia and contributed great wealth to the Empire, funding the imperial army and navy.
Because of the extent and power of the Spanish Empire, the coin had become the first world currency by the late 19th century. Large shipments of Pieces of Eight attracted sea pirates, and the fabled pirate buried treasures were typically imagined as chests full of the coins and other valuables.
The Piece of Eight was legal tender in the United States until 1857, even though the U.S. started minting its own coins in 1792. It was the origin of the 'bit' as a slang term for a unit of currency. 'Two bits' became a quarter dollar (25 cents), etc. Stock prices were listed in eigths of a dollar on the New York Stock Exchange until 1997.
Spanish dollar
Pieces of Eight (BBC)
What are pieces-of-eight?