shader
In 3D computer graphics, a shader is a program used to determine the final surface properties of an object or image. This can include arbitrarily complex descriptions of light absorption and diffusion, texture mapping, reflection and refraction, shadowing, surface displacement and post-processing effects.

Programmable shaders were introduced in Pixar's RenderMan by Rob Cook, which currently defines five major types:

Light source shaders calculate the color of the light emitted from a point on the light source towards a point on the surface being illuminated.
Surface shaders are used to model the optical properties of the object being illuminated. They output the final color and position of the point being illuminated by taking into account the incoming light and the physical properties of the object.
Displacement shaders manipulate the surface's geometry independent of its color.
Volume shaders manipulate the color of a light as it passes through a volume. They are used to create effects like fog.
Imager shaders describe a color transformation to final pixel values. This is much like an image filter, however the imager shader operates on prequantized data, which typically has a greater dynamic range than can be displayed on the output device.