The Collaborative International Dictionary of English v.0.48:

Decipium \De*cip"i*um\, n. [NL., fr. L. decipere to deceive.]
   (Chem.)
   A supposed rare element, said to be associated with cerium,
   yttrium, etc., in the mineral samarskite, and more recently
   called samarium. Symbol Dp. See Samarium.
   [1913 Webster]