A cut piece of clear plastic or glass in the form of a triangle or other geometric shape is called a prism.

Here's why:

* Prism: In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The most common shape is a triangular prism, but prisms can have other geometric shapes too.

* Refraction: When light passes from one medium to another (like air to glass), it changes direction. This bending of light is called refraction.

* Dispersion: Prisms are particularly useful because they can split white light into its constituent colors (the rainbow) through a process called dispersion. This happens because each color of light is refracted at a slightly different angle.

So, while you could technically call it a "triangle of plastic/glass", the more accurate term is a prism.