/ ˈkærət; NAmE / noun ( abbr. ct )
1.
a unit for measuring the weight of diamonds and other precious stones , equal to 200 milligrams
2.
( especially BrE ) ( NAmE usually karat ) a unit for measuring how pure gold is. The purest gold is 24 carats :
an 18-carat gold ring
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WORD ORIGIN
late Middle English (in sense 2): from French , from Italian carato , from Arabic qīrāṭ (a unit of weight), from Greek keration fruit of the carob tree (also denoting a unit of weight), diminutive of keras horn, with reference to the elongated seed pod of the carob.