From AMS Glossary
The initial leader of a lightning discharge; an intermittently advancing column of high ionization and charge that establishes the channel for a first return stroke.
The peculiar characteristic of this type of leader is its stepwise growth at intervals of about 50– 100 μs. The velocity of growth during the brief intervals of advance, each only about 1 μs in duration, is quite high (about 5 × 107 m s-1), but the long stationary phases reduce its effective speed to only about 5 × 105 m s-1. To help explain its mode of advance, the concept of a pilot streamer was originally suggested, but has been supplanted by analogy to recent work on long laboratory sparks.