The Collaborative International Dictionary of English v.0.48:

Dy \Dy\ n.
   the chemical symbol for dysprosium, a rare earth element of
   atomic number 66.

   Syn: dysprosium.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Dy
    n 1: a trivalent metallic element of the rare earth group; forms
         compounds that are highly magnetic [syn: dysprosium,
         Dy, atomic number 66]