Plain English Meaning
How far you can clearly see and recognize things through the air. It is reported as a distance — for example, 10 miles, 3 miles, or half a mile — based on how clear or hazy the atmosphere is.
Full Definition
The greatest horizontal distance at which prominent unlit objects can be seen and identified by day, and prominent lighted objects can be seen and identified by night, as determined by atmospheric conditions and reported in statute miles or fractions thereof.
Context Anchor
You will see visibility in aviation weather reports, preflight planning, airport observations, and decisions about whether conditions are good enough to fly visually.
Derivation
From Latin visibilis, 'able to be seen,' from videre, 'to see.' In it is given a precise meaning: not just whether something can be seen, but the measured distance at which it can be identified.