The piastre or piaster is any of a number of units of currency. The term originates from the Italian for "thin metal plate". The name was applied to Spanish and Hispanic American pieces of eight, or pesos, by Venetian traders in the Levant in the 16th century.

The Spanish dollar, also known as the piece of eight, is a silver coin of approximately 38 mm (1.5 in) diameter worth eight Spanish reales. It was minted in the Spanish Empire following a monetary reform in 1497 with content 25.563 g (0.8219 ozt) fine silver. It was widely used as the first international currency because of its uniformity in standard and milling characteristics. Some countries countermarked the Spanish dollar so it could be used as their local currency.