Prism

·noun A form the planes of which are parallel to the vertical axis. ·see Form, ·noun, 13.

II. Prism ·noun A solid whose bases or ends are any similar, equal, and parallel plane figures, and whose sides are parallelograms.

III. Prism ·noun A transparent body, with usually three rectangular plane faces or sides, and two equal and parallel triangular ends or bases;

— used in experiments on refraction, dispersion, ·etc.