From International Dictionary of Marine Aids to Navigation
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1. That property of the atmosphere which determines the ability of an observer to see and identify prominent objects by day, or lights or lighted objects by night.

2. A measure of this property of the atmosphere expressed in units of distance.

3. Commonly used for "meteorological visibility".

Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)

International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM