The Collaborative International Dictionary of English v.0.48:

Indium \In"di*um\, n. [NL. See Indigo.] (Chem.)
   A rare metallic element, of atomic number 49, discovered in
   certain ores of zinc, by means of its characteristic spectrum
   of two indigo blue lines; hence, its name. In appearance it
   resembles zinc, being white or lead gray, soft, malleable and
   easily fusible, but in its chemical relation it resembles
   aluminium or gallium (valence +3). Symbol In. Atomic weight,
   114.82.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

indium
    n 1: a rare soft silvery metallic element; occurs in small
         quantities in sphalerite [syn: indium, In, atomic
         number 49]