1. Cross-section of a prism by a plane normal to the refracting edge. The section is bounded by straight lines (usually three).
2. In lighthouse practice, the term is also used for a cross-section of a transparent optical element bounded by three straight or slightly curved sides, approximating to a triangle, but of such shape that incident parallel rays of a particular wavelength are focussed after refraction or reflection (or both) by the section.
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