Meaning of phantom circuit

phan'tom cir'cuit

Pronunciation: [key]
— Elect. Elect.
  1. a circuit derived from two suitably arranged pairs of wires, each pair being a circuitand also acting as one half of an additional derived circuit, the entire system providing the capabilities of three circuits while requiring wires for only two.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.