In electromagnetism, a leader is a hot, highly conductive channel of plasma that plays a critical part during dielectric breakdown within a long electric spark.
Breakdown initially begins with the formation of corona discharges near the electrode with the highest electrical stress.
If the electrical field is further increased, longer length cold discharges (called streamers or burst corona) sporadically form near the stressed electrode.
The tip of the conductive leader now forms a new region from which streamers can extend even further into the gap.
Although leaders are most often associated with the initial formative stages of a lightning stroke, they are characteristic of the development of all long sparks.