The Collaborative International Dictionary of English v.0.48:

Unipolar \U`ni*po"lar\, a. [Uni- + polar.]
   1. (Physics) Having, or acting by means of, one pole only.
      [1913 Webster]

   2. (Anat.) Having but one pole or process; -- applied to
      those ganglionic nerve cells which have but one radiating
      process; -- opposed to multipolar.
      [1913 Webster]

   Unipolar induction (Elec.), induction, as in a conducting
      circuit, by only one pole of a magnet.

   Unipolar stimulation (Physiol.), the simulation sometimes
      produced when one electrode of an induction apparatus is
      applied to a nerve; -- called also unipolar induction
      action. --Du Bois-Reymond.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unipolar
    adj 1: having a single pole [ant: bipolar]