Plain English Meaning
Something has gone wrong with the airplane's electrical power. The part that makes electricity, the battery, or the wiring is no longer working properly, so things that need electricity may stop working or behave strangely.
Full Definition
In-flight failures or abnormalities in the airplane's electrical generating, distribution, or storage components — including the alternator or generator, battery, voltage regulator, wiring, and associated circuits — that reduce or eliminate electrical power to instruments, avionics, lighting, and other electrically driven systems.
Context Anchor
Encountered in emergency training when discussing how to handle airplane problems that may affect radios, lights, instruments, flaps, landing gear, or other equipment powered by electricity.