Karat or Carat is a measure of fractional fineness that along with 'mass' and 'total mass' defines the purity of gold. It's only used for gold and the system is based on how many parts per 24-unit whole are pure gold.
For example, 24-karat gold is considered 24 parts or 100% pure - even though it’s impossible to attain 100% purity in gold. 24-karat is the designation given to gold with a millesimal fineness of 99.95% and above for commercial purposes.
According to the karat system, 18-karat gold is 18 parts gold and 6 parts other metal in an alloy that's 75% pure, while 12-karat is 12 parts gold and 12 parts other metal and so on. Today the karat system is often replaced or complemented by the more precise millesimal fineness system which is calculated per 1000 units.