The Collaborative International Dictionary of English v.0.48:

Brocken specter \Brock"en spec"ter\ or Brocken spectre \Brock"en
spec"tre\ [Trans. of G. Brockengespenst.]
   A mountain specter (which see), esp. that observed on the
   Brocken, in the Harz Mountains.
   [Webster 1913 Suppl.]