Final answer:
The term for the initial leading stroke of electrons moving downward in a lightning strike is a step leader, which initiates the strike by ionizing the air to create a conductive path.
Step-by-step explanation:
The term for the initial leading stroke of electrons that move downward from a cloud during a lightning strike is the step leader.
The initial leading stroke of electrons that move downward from a cloud during a lightning strike is called a step leader.
In a lightning strike, the discharge process begins with a preliminary breakdown within the cloud. This starts with the formation of a step leader, which is a channel of charged particles that initiates the lightning strike. The step leader progresses towards the ground in a series of rapid jumps, setting the stage for the lightning strike by ionizing the air and creating a conductive path. As it moves, it is almost invisible and proceeds in a stepwise fashion, hence its name.
This contrasts with the dart leader, which may follow in subsequent strikes reusing the established path of the initial step leader, and the return stroke, which is the upward-moving bright flash that we commonly associate with lightning, resulting from the discharge of electrons traveling upwards once the step leader makes contact with a positively charged channel coming from the ground.