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- A solidus (the Latin word for solid) was originally a gold coin issued by the Romans. It was introduced by Constantine I in the AD 309–10, and was used through the Byzantine Empire until the 10th century AD. The coin replaced the aureus as the main gold coin of the Roman Empire. Fractions of the solidus known as semissis (half-solidi) and tremissis (one-third solidi) were also produced.