From AMS Glossary
A relatively slow-moving, nonluminous lightning streamer, the existence of which has been postulated but not verified, to provide a physical explanation for the observed intermittent mode of advance of a stepped leader as it initiates a cloud-to-ground lightning discharge.
Whereas the stepped leader descends at an average speed of the order of 105 m s-1 during its downward motion, it advances only about 50 m at a time with a speed greater than average and then pauses for 50–100 μs before resuming its downward movement. The average downward speed has been associated with an invisible streamer, the pilot streamer, which is postulated to descend at a uniform speed only slightly in excess of the ionizing speed of electrons in air and lay down a trail of weak residual ionization along which the stepped leader moves very rapidly in a pulsating manner. The idea of a pilot leader has been supplanted by more modern theory based on studies of sparks over long distances in the laboratory.