Source : Webster's Revised Unabridged Dictionary (1913)

Bifilar \Bi*fi"lar\, a. [Pref. bi- + filar.]
   Two-threaded; involving the use of two threads; as, bifilar
   suspension; a bifilar balance.

   {Bifilar micrometer} (often called {a bifilar}), an
      instrument form measuring minute distances or angles by
      means of two very minute threads (usually spider lines),
      one of which, at least, is movable; -- more commonly
      called a {filar micrometer}.