Plain English Meaning
A small gauge that tells you whether your aircraft's electrical system is charging the battery or draining it.
Full Definition
A cockpit instrument that shows the flow of electrical current, in amperes, between the alternator (or generator) and the battery. A positive reading means the alternator is charging the battery; a negative reading means the battery is discharging because it is supplying the electrical load on its own.
Context Anchor
Seen during engine starting and after-start checks, when the pilot confirms that the airplane’s electrical system is working normally.
Derivation
From 'ampere' (the unit of electrical current, named after French physicist André-Marie Ampère) plus '-' (a device that measures). Literally, a meter that measures amperes.