From Glossary of Meteorology
(Also called continuous leader.) The leader which, after the first stroke, typically initiates each succeeding stroke of a multiple-stroke flash lightning. (The first stroke is initiated by a stepped leader.)
The dart leader derives its name from its appearance on photographs taken with streak cameras. The dart leader's brightest luminosity is at its tip which is tens of meters in length, propagating downward at about 107 m s-1. In contrast to stepped leaders, dart leaders do not typically exhibit branching because the previously established channel's low gas density and residual ionization provide a more favorable path for this leader than do any alternative ones.
Chalmers, J. A. 1957. Atmospheric Electricity. p. 239.