How do you define a Shader in Unity?

A shader in Unity is fundamentally a program that provides instructions to the Graphics Processing Unit (GPU) on how to render the visual appearance of surfaces. This includes determining various aspects such as colors, textures, lighting effects, and how surfaces interact with light in the scene. Shaders enable developers to create complex visual effects, from simple color applications to advanced effects like reflections, refractions, and shadows. By writing shaders, developers can tailor the rendering pipeline to achieve specific artistic goals and optimize performance.

In contrast, a script for handling game logic is typically written in C# and controls the behavior of game objects, while a visual element in the editor relates to Unity's user interface and does not define rendering behaviors. A 3D model format refers to the structural data behind 3D objects rather than the rendering techniques applied to them. Thus, the definition of a shader as a program guiding the GPU is distinct and accurate, capturing its primary function within Unity.