From International Dictionary of Marine Aids to Navigation
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A transparent optical element, usually of glass or plastic, bounded by at least two planes intersecting at a refracting edge.

The angle formed at the refracting edge, and in a plane normal to it, constitutes the prism angle.

Incident light rays on a prism are deviated into new directions as the result of refraction or total internal reflection or both.

Note: In lighthouse practice, the word "prism" is also loosely used for any dioptric or catadioptric ring.

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