The carat is a unit of mass used for gems. The word derives from the Arabic kirat. In the past, different countries each had their own carat. However, in 1907 the metric carat of 200 milligrams was adopted, which is now universally used today.
The carat (spelt karat in the U.S., to distinguish it from the unit of mass above) is also a measure of the purity of precious metals such as gold. One carat in this sense is one-twenty-fourth purity. Therefore 24 carat gold is pure gold, 12 carat gold is 50% purity, etc.