Shader
A shader is a general industry term for a system or network of shading elements that help define how an object looks and reacts to light when needed. Shading network is a more accurate description in Maya because shaders in Maya consist of a network of nodes that each have specialized functions. Because materials can be used, generally, as nodes inside a shading network they have a separate label compared to other 3D rendering applications. The quality or look of a surface is generally attributed to the shader or shading network that is assigned to this surface. See also Material, Shading Group and Surface Shader.
Короче, это совокупность материалов, текстур, свойств, накладываемых на обьект и их взаимодействия...
Примерно так.