Plain English Meaning
Little fans that blow air over the airplane's electronic equipment to stop it from overheating.
Full Definition
Small electric fans installed in the avionics bay or instrument panel area to draw heat away from radios, navigation equipment, and other electronic units, keeping them within their designed operating temperature range.
Context Anchor
A pilot may check these during preflight by listening for fan operation, checking for a warning, or making sure cooling openings are not blocked, depending on the airplane.
Derivation
Avionics' comes from blending '' and 'electronics' — the electronic used in . Knowing this makes it that an avionics cooling fan is specifically for cooling the airplane's electronics, not the engine or .