A small program the GPU runs for every pixel on every frame, computing color directly instead of compositing images; how a screen gets gradients that drift, glass that refracts, and grain that never tiles.
The hero background is a shader, noise over a two-stop gradient. Exporting it as video would cost megabytes for something the GPU does in a kilobyte.
The word comes from film rendering, where a shader decided how a surface answered light, and the name held even as the programs learned to do everything else. That origin is the right way to think about one: a shader describes a material, not a clip. A video of a gradient loops, visibly, forever; the shader is the gradient, live, and never repeats. But computed-per-frame means moving-per-frame, and ambient motion is the most expensive kind. Spend a shader where the surface is the point, honor reduced-motion by resting it on a still frame, and never let the material upstage the words set on it.