Pronunciation: (priz'um), [key]
1. Optics.a transparent solid body, often having triangular bases, used for dispersing light into a spectrum or for reflecting rays of light.
2. Geom.a solid having bases or ends that are parallel, congruent polygons and sides that are parallelograms.
3. Crystall.a form having faces parallel to the vertical axis and intersecting the horizontal axes.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.