/pawr"oh/ , Optics.
an isosceles, right-triangular prism in which light entering one half of the hypotenuse face is reflected at the two short sides and is reversed in orientation when it leaves the other half of the hypotenuse: used in 90°-oriented pairs in binoculars to increase the length of the optical path and to erect the final image.
[ named after Ignazio Porro (1801-75), Italian engineer ]