A three-dimensional figure whose bases or ends have the same size and shape and are parallel to one another, and each of whose sides is a rectangle. A transparent solid of this form, typically of glass and usually with triangular ends, used for separating white light passed through it into a spectrum, or for reflecting light beams. And, ground-glass objects such as those used as components of crystal chandeliers. Also, used metaphorically to refer to a medium that misrepresents whatever is seen through it.Also see optical and triangle.