a famous Indian diamond which has a history going back to the 14th century. It passed into British possession on the annexation of Punjab in 1849, and was set in the queen’s state crown for the coronation of George VI (1937).
from Persian kōh-i nūr 'mountain of light'
Koh-i-noor in other Oxford dictionaries
Definition of Koh-i-noor in the US English dictionary