Plain English Meaning
The internal wiring inside a magneto that the ignition switch uses to turn it off. When the switch is OFF, this wire is grounded and the magneto cannot fire. When the switch is ON, the wire is no longer grounded and the magneto can produce sparks.
Full Definition
The low-voltage portion of an aircraft magneto's wiring that, when grounded by a closed ignition switch, prevents the magneto from producing a spark. When the primary circuit is open (ignition switch ON), the magneto generates and delivers high-voltage current through the secondary circuit to the spark plugs.
Context Anchor
Encountered during hand propping, ignition switch checks, and discussions of why a propeller can be dangerous even when the cockpit switch appears to be OFF.