The solidus was the Byzantine gold coin that replaced the Roman aureus.
Emperor Constantine the Great introduced the solidus around 309 CE as part of a monetary reform. It weighed about 4.5 grams of gold and was struck to a high and relatively consistent standard. The reform helped restore confidence in Roman gold money after decades of political instability and debasement.
The solidus remained important for centuries. In the Eastern Roman, or Byzantine, Empire it became a principal coin for state payments, taxation and international trade. Its reliability made it influential beyond imperial borders, and later European gold coins were sometimes described using related names.
The coin is sometimes called the nomisma in Greek-language Byzantine contexts. Its smaller fraction was the semissis, while the tremissis was a third of a solidus. The solidus should not be confused with the much earlier aureus, which it superseded in the Roman monetary system.