Karat is a variant of carat. First attested in English in the mid-15th century, the word carat came from Middle French , in turn derived either from Italian  or Medieval Latin . These were borrowed into Medieval Europe from the Arabic  meaning "fruit of the carob tree", also "weight of 5 grains", () and was a unit of mass though it was probably not used to measure gold in classical times. The Arabic term ultimately originates from the Greek  () meaning carob seed (literally "small horn")  (diminutive of  – , "horn").
In 309 CE, Roman Emperor Constantine I began to mint a new gold coin solidus that was  of a libra (Roman pound) of gold equal to a mass of 24 siliquae, where each siliqua (or carat) was  of a libra. This is believed to be the origin of the value of the karat.