The Collaborative International Dictionary of English v.0.48:

Decembrist \De*cem"brist\, n. (Russian Hist.)
   One of those who conspired for constitutional government
   against the Emperor Nicholas on his accession to the throne
   at the death of Alexander I., in December, 1825; -- called
   also Dekabrist.

         He recalls the history of the decembrists . . . that
         gallant band of revolutionists.          --G. Kennan.
   [Webster 1913 Suppl.]