My optician asked me if I need prism in my lenses when I order eyeglasses last week, what’s prism? I have never heard about that! I have worn out 2 pairs of eyeglasses, and there’s no such prism in my lenses, Why do some progressive lenses need it? And does everyone need prism for their prescription eyeglasses?

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use "prism" usually refers to this type. Some types of optical prism are not in fact in the shape of geometric prisms. Prisms are typically made out of glass, but can be made from any material that is transparent to the wavelengths for which they are designed.