Definition
The amount of visible light falling on a surface, measured in foot-candles (or lux in metric units). One foot-candle equals the light produced by a standard candle at a distance of one foot.
Plain English
How much light is hitting a surface. It's a measure of brightness on the thing being lit, not the brightness of the light source itself.
Context Anchor
Pilots encounter this term when discussing cockpit lighting, instrument-panel lighting, airport lighting, ramp lighting, and night operations.
Derivation
From the Latin 'illuminare,' meaning 'to light up' or 'to make bright.' The aviation use stays close to this original meaning -- it's about how much light is being delivered onto a surface.
Why Pilots Care
Proper illumination of runways and instruments is critical for safe night flying and maintaining situational awareness during all phases of flight.
Intuition Check
Illumination does not always mean the object is making its own light. It can also mean light shining onto the object or area so the pilot can see it.
Example Sentence 1
The cockpit illumination was dimmed during the night flight to preserve the pilot's night vision.
Example Sentence 2
Decreased cockpit illumination at night helps preserve the pilot's night vision for scanning outside the aircraft.