Plain English Meaning
The instruction books for the electronic gear in your aircraft. They tell you how each piece of equipment works, what it can and can't do, and how to use it correctly.
Full Definition
Manufacturer-published documents that describe the operation, limitations, and procedures for the electronic equipment installed in an aircraft, such as GPS units, autopilots, transponders, and integrated flight displays. They supplement the Pilot's Operating Handbook and are typically required to be carried or accessible when the equipment is installed.
Context Anchor
A pilot uses avionics manuals during training, before flying an unfamiliar aircraft, or whenever a cockpit electronic system has features, alerts, or operating steps the pilot does not fully understand.
Derivation
Avionics combines and electronics, a term coined in the 1940s as electronic equipment became in aircraft. So avionics manuals are simply the manuals for the aviation-electronics gear.