PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)  , in geometry a solid enclosed by plane surfaces, two of which, termed the ends, are parallel, equal, similar and similarly situated polygons, and the faces connecting the ends are parallelograms, equal in number to the sides of the polygon . If the faces be perpendicular to the ends the prism is a " right prism," and the faces are rectangles; otherwise the prism is " oblique." The axis is the line joining the centres of the ends . It may be generated by moving a plane (corresponding to an end or base) parallel to itself . A prismoid differs from a prism in having for its ends two dissimilar parallel figures . For illustrations see CRYSTALLOGRAPHY, and for the mensuration see that article . In optics the word denotes a triangular prism, i.e. one having a triangle for base, used to decompose white light .