Plain English Meaning
One switch that turns the battery and the alternator on or off at the same time. It only has two settings — on or off — so you cannot run the battery without also running the alternator, or the other way around.
Full Definition
A single rocker-style cockpit switch that controls both the battery master circuit and the alternator field circuit together, with two operating positions: ON (both battery and alternator energized) and OFF (both de-energized). Unlike a split master switch, the two halves cannot be operated independently — moving the rocker controls both circuits simultaneously.
Context Anchor
Seen in electrical-system procedures, especially when managing an alternator failure or conserving battery power in .