First used in 1901 in The First Men in the Moon by H. G. Wells. It was named after the fictional Dr. Cavor. The initial use was capitalized, but later authors borrowing the concept sometimes changed it to lowercase.
In the first use, the metal acted as a gravity shield; after the metal was cooled, objects contained within it were no longer subject to gravity. Some later uses, e.g., Alan Moore's, imagine the substance to simply have a negative gravitational mass.