The Collaborative International Dictionary of English v.0.48:

Pelorus \Pe*lo"rus\, n. [After Pelorus, said to have been
   Hannibal's pilot when he left Italy.] (Navig.)
   An instrument similar to a mariner's compass, but without
   magnetic needles, and having two sight vanes by which
   bearings are taken, esp. such as cannot be taken by the
   compass.
   [Webster 1913 Suppl.]