The Collaborative International Dictionary of English v.0.48:

Bifilar \Bi*fi"lar\, a. [Pref. bi- + filar.]
   Two-threaded; involving the use of two threads; as, bifilar
   suspension; a bifilar balance.
   [1913 Webster]

   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.
      [1913 Webster]