The solidus was the gold coin introduced by Constantine the Great around 309 CE. It replaced the older aureus as the Roman Empire’s principal gold coin and remained remarkably stable for centuries.
A solidus weighed about 4.5 grams of gold, although standards and actual weights could vary. Its reliability made it especially useful for imperial payments, military salaries, taxation, and long-distance trade.
The coin continued in the Eastern Roman, or Byzantine, Empire, where it was often called the nomisma. Byzantine gold coins based on the solidus circulated widely and became trusted international money.
The solidus’s long life is a major contrast with many ancient currencies, whose metal content and purchasing power changed sharply. In medieval Europe, the word “shilling” and related monetary terms were influenced by the solidus, even though later currencies were not direct physical copies of the Roman coin.