Visibility

(1) The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye, and (a) in the daytime, a prominent dark object against the sky at the horizon, and (b) at night, a known, preferably unfocused moderately intense light source.

Visibility The greatest distance that prominent objects can be seen and identified by unaided, normal eyes.

The distance at which a given standard object can be seen and identified with the unaided eye

how well-placed your web site is in the search engines for relevant keyword searches. Also see "Invisible Web."