Plain English Meaning

It is the position on the ignition key — past OFF, R, L, and BOTH — that you turn to and hold to spin the engine over and light it off. As soon as the engine catches, you let go and the key snaps back to BOTH.

Full Definition

In the context of an aircraft ignition system, START refers to the momentary ignition switch position that engages the starter motor and energizes both magnetos (often through an impulse coupling or starter vibrator on the left magneto) to crank the engine and produce the retarded, boosted spark needed for engine start. When the key is released, it springs back to the BOTH position for operation.