Bifilar Winding or Parallel Winding, is a coil with two windings, with the wire of each winding alongside the other, matching turn for turn. The two wires are wound simultaneously with the turns of each winding being contiguous. This is done to suppress back EMF. Special Reed Switch based Reed Relays use such windings on the coil for low level signal switching and to reduce circuit noise.
A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. In engineering, the word bifilar describes wire which is made of two filaments or strands. It is commonly used to denote special types of winding wire for transformers. Wire can be purchased in bifilar form, usually as different colored enameled wire bonded together. For three strands, the term trifilar coil is used.