1. The maximum distance (usually horizontal) at which a given object can be seen by day in any particular circumstances, as limited by the atmospheric transmission. The distance is such that the contrast of the object with its background is reduced by the atmosphere to the contrast threshold value for the observer (2-1-300).
2. Loosely used as a synonym for "visibility" (in sense 2 of 2-1 -275). This use of the term is deprecated.
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