Ducat
1. A gold coin of varying value, formerly in use in most European countries; said to be worth about 9s. 4d.
2. A money of account in the Venetian republic.
3. (loosely) A piece of money.
The etymological origin of the name is from Medieval Latin ducatus, 12th century Italian ducato, initially meaning “duke’s coin” or a “duchy’s coin”. According to the OED, the first recorded use in English was around 1384 by Chaucer.

Venetian ducat from the time of Doge Michele Steno, 1400-1413
Originally used as the name of a silver coin issued in 1140 by Roger II of Sicily the ducat became a trading coin largely due to its use by Venice. The first gold ducat, also called zecchino d’oro, was struck at Venice in 1284 under the Doge John Dandolo. Subsequently many European states issued their own ducats (and fractions of ducats) usually of gold, but sometimes of silver. As always there is a lot more information on Wikipedia.