Meaning

noun a unit of weight for precious stones = 200 mg

noun the unit of measurement for the proportion of gold in an alloy; 18-karat gold is 75% gold; 24-karat gold is pure gold

originFrom Middle French carat, from Italian carato, from Arabic قِيرَاط (qīrāṭ, “carat, similarly small units such as inches”), from Ancient Greek κεράτιον (kerátion, “hornlet, carob seed”), from κέρας (kéras, “horn”) + -ιον (-ion, diminutive suffix). Doublet of quilate and keratin.

Etymology from Wiktionary, CC BY-SA 3.0