What is a shader in Unity?

A shader in Unity is a program that runs on the GPU (Graphics Processing Unit). This specialized program is responsible for computing rendering effects on graphics hardware. Shaders play a crucial role in defining how the surface of a 3D model interacts with light, which determines how textures and colors are applied, how lighting effects are simulated, and how the final image is rendered on the screen.

By executing on the GPU, shaders allow for efficient parallel processing of rendering calculations, significantly improving performance in graphics-heavy applications such as video games. The GPU is designed to handle these tasks much more efficiently than a CPU (Central Processing Unit), which is why shaders are optimized for GPU execution.

Other options like a program that runs on the CPU are not accurate since shaders utilize the parallel architecture of GPUs for rendering tasks. Similarly, while shaders can manipulate textures, they are not types of textures themselves; rather, they are programs that instruct how textures are applied. Moreover, a framework for game design refers to the overall structure and tools available for game development, which does not specifically pertain to the function or definition of a shader.