Meaning of pentaprism

pen•ta•prism

Pronunciation: (pen'tu-priz"um), [key]
— n.
  1. a prism that has five faces, a pair of which are at 90° to each other; a ray entering one of the pair emerges from the other at an angle of 90° to its original direction: used esp. in single-lens reflex cameras to reverse images laterally and reflect them to the viewfinder.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.