From International Dictionary of Marine Aids to Navigation
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1. A transparent optically refracting element, usually of glass or plastic, bounded by two rotationally-symmetrical surfaces having a common axis. These surfaces are usually spherical, but may have other rotationally-symmetrical forms. A lens is designed to have a focus (2-1-125).

2. Frequently used in a more general sense for an assembly of optical elements which have a common focus. For examples, see 2-2-085, 2-2-090, 2-2-100, 2-2-140, 2-2-150.

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