Erbium, er′bi-um, n. a rare metal, the compounds of which are present in the mineral gadolinite, found at Ytterby in Sweden.

The first of the rare earth minerals was discovered in 1794 by J. Gadolin and was named gadolinite from its discoverer.

It occurs in certain rare minerals, as cerite, gadolinite, orthite, etc., and was so named from the difficulty of separating it from cerium, didymium, and other rare elements with which it is usually associated.