What does "carat" mean?

Carat: A twenty-fourth part; -- a term used in estimating the proportionate fineness of gold. Note: A mass of metal is said to be so many carats fine, according to the number of twenty-fourths of pure gold which it contains; as, 22 carats fine (goldsmith's standard) = 22 parts of gold, 1 of copper, and 1 of silver.

Sources