·add. ·noun A metallic element of the rare earth group, found in gadolinite and some other minerals. Symbol, Er; at. wt. 167.4. Its salts are rose-colored and give characteristic spectra.
II. Erbium ·noun A rare metallic element associated with several other rare elements in the mineral gadolinite from Ytterby in Sweden. Symbol Er. Atomic weight 165.9. Its salts are rose-colored and give characteristic spectra. Its sesquioxide is called erbia.