What is "decipium"

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.

n. (context obsolete English) Proposed name for a chemical element isolated from samarskite, later found to be a mixture of samarium and other rare earth elements.

Decipium was the proposed name for a new chemical element isolated by Marc Delafontaine from the mineral samarskite. He published his discovery in 1878 and a follow-up paper in 1881. Later it was shown that decipium was a mixture of samarium and other rare earth elements.