Etymology
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prism (n.)

1560s, in geometry, "a solid whose bases or ends are any similar, equal, and parallel plane polygons, and whose sides are parallelograms" (not always triangular), from Late Latin prisma, from Greek prisma "a geometrical prism, trilateral column," (Euclid), literally "something sawed (as a block of wood), sawdust," from prizein, priein "to saw" (related to prion "a saw"), which is of uncertain origin. Euclid chose the word, apparently, on the image of a column with the sides sawn off.

Specific sense in optics, "an instrument (usually triangular) with well-polished sides of glass, quartz, etc., which refracts light and spreads it in a spectrum," is attested from 1610s.

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Definitions of prism

prism (n.)
a polyhedron with two congruent and parallel faces (the bases) and whose lateral faces are parallelograms;
prism (n.)
optical device having a triangular shape and made of glass or quartz; used to deviate a beam or invert an image;
Synonyms: optical prism
From wordnet.princeton.edu