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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," 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 consisting of a transparent medium so arranged that the surfaces which receive and transmit light form an angle with each other, usually of a triangular form, with polished sides which meet in three parallel lines," is attested from 1610s.

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