The Collaborative International Dictionary of English v.0.48:

Dysprosium \Dys*pro"si*um\, n. [NL., fr. Gr. dyspro`sitos hard
   to get at.] (Chem.)
   An element of the rare earth-group. Symbol Dy; at. wt.,
   162.5.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

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

The Elements (07Nov00):

dysprosium
Symbol: Dy
Atomic number: 66
Atomic weight: 162.50
Metallic with a bright silvery-white lustre. Dysprosium belongs to the
lanthanoids. It is relatively stable in air at room temperatures, it
will
however dissolve in mineral acids, evolving hydrogen. It is found in
from rare-earth minerals. There are seven natural isotopes of
dysprosium,
and eight radioisotopes, Dy-154 being the most stable with a half-life
of
3*10^6 years. Dysprosium is used as a neutron absorber in nuclear
fission
reactions, and in compact disks. It was discovered by Paul Emile Lecoq
de
Boisbaudran in 1886 in France. Its name comes from the Greek word
dysprositos, which means hard to obtain.