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Julian solidus, ca. 361.

Avitus tremissis, one-third of a solidus, ca. 456.

A solidus (the Latin word for solid) was originally a gold coin issued by the Romans. It was introduced by Constantine I in 309–10, and was used through the Byzantine Empire until the 10th century. The coin replaced the aureus as the main gold coin of the Roman Empire.

The name solidus had previously been used by Diocletian (284–305) for the gold coin that he introduced, which is different from the solidus introduced by Constantine. The coin was struck at a theoretical value of 1/72 of a Roman pound (about 4.5 grams). Solidi were wider and thinner than the Aureus, with the exception of some lower quality issues from the Byzantine Empire. The weight and fineness of the solidus remained relatively constant throughout its long production. Fractions of the solidus known as semissis (half-solidi) and tremissis (one-third solidi) were also produced.

The word soldier is ultimately derived from solidus, referring to the solidi with which soldiers were paid.

Variations on the word solidus gave rise to a number of currency units:

To this day, sou is used as slang for money, as in sans le sou. "I'm broke", "without money". It is also a slang term for the Canadian cent (standard French, cent).

Sou of copper, coined 1767 for Louis XV of France

The name of the medieval Italian soldo (plural soldi) was derived from solidus. This word is still in common use today in Italy in its plural soldi with the same meaning that the english equivalent money.

The name of the medieval Spanish sueldo (which also means salary) was derived from solidus, which is also used in the Philippines as Suweldo. Subsequently the name of the Peruvian sol (more formally sol de oro intended to mean gold solidus) was derived from this name although, because sol actually means sun in Spanish (from the Latin sol), the etymology of the currency's name is commonly misunderstood. To complicate matters, the Sun God was a foremost figure of the Incan Empire that reigned in what is now Peru.

Until decimalisation in the United Kingdom in 1971, the abbreviation s., from solidus, was used to represent shillings, just as d. and £, from denarius and Libra, were respectively used to represent pence and pounds, leading to the abbreviation "£sd".

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Solidus

• Roman currency
• Byzantine coinage
• Solidus (punctuation)
• Slash (punctuation) (also called a solidus)