Plain English Meaning
The part of the aircraft that makes electricity has stopped working. The battery is now the only source of electrical power, and it will run out unless the pilot turns off equipment that is not essential.
Full Definition
A condition in which the aircraft's primary electrical generating device — either an alternator or a generator — stops producing electrical power. When this occurs, the battery becomes the sole source of electrical energy and will deplete within a limited time, after which all electrically powered equipment will cease to function unless load is reduced or power is restored.
Context Anchor
Encountered during instrument flying, , and , especially when a - , discharge indication, or loss of electrical equipment appears.