Solidus coin

A solidus (the Latin word for solid) was initially a gold coin issued by the Romans.

Julian solidus, ca. 361.It was introduced by Constantine I in the AD 309–10, and was used from beginning to end the Byzantine Empire until the 10th century AD. The coin replaced the aureus as the major gold coin of the Roman Empire.

The name solidus had beforehand 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 worth of 1/72 of a Roman pound (about 4.5 grams). Solidi were wider and thinner than the aureus, with the exclusion of some dumpy issues from the Byzantine Empire. The weight and fineness of the solidus remained moderately constant throughout its long production, with few exceptions. Fractions of the solidus recognized as semisses (half-solidi) and tremissis (one-third solidi) were also produced.

Avitus tremissis, ca. 456.The word soldier is eventually derived from solidus, referring to the solidi with which soldiers were paid.