The solidus (Latin 'solid'; pl. solidi) or nomisma (Greek: νόμισμα, nómisma, lit. 'coin') was a highly pure gold coin issued in the Late Roman Empire and Byzantine Empire. The early 4th century saw the solidus introduced in mintage as a successor to the aureus, which was permanently replaced thereafter by the new coin, whose weight of about 4.5 grams remained relatively constant for seven centuries. In the Byzantine Empire, the solidus or nomisma remained a highly pure gold coin until the 11th century, when several Byzantine emperors began to strike the coin with less and less gold. The nomisma was finally abolished by Alexius I in 1092, who replaced it with the hyperpyron, which also came to be known as a "bezant". The Byzantine solidus also inspired the originally slightly less pure dinar issued by the Muslim Caliphate. In Western Europe, the solidus was the main gold coin of commerce from late Roman times to Pepin the Short's currency reform, which introduced the silver-based pound/shilling/penny system.

In Late Antiquity and the Middle Ages, the solidus also functioned as a unit of weight equal to 1⁄72 Roman pound (approximately 4.5 grams).