Scissor test
The GPU stage that throws away every fragment outside a given rectangle before it is drawn — cheaper than rendering everything and clipping afterwards.
The scissor test sits late in the fragment pipeline, after rasterization but typically before the depth and stencil tests. You define an axis-aligned rectangle in window coordinates, and anything falling outside it is discarded without being shaded or written. It is set with glScissor in OpenGL or a scissor VkRect2D in Vulkan.
It is used to clip a UI against a panel, confine a split screen, clear only part of a buffer, or let a light update just its own region. The test is easily confused with the viewport, but they do different jobs: the viewport transforms coordinates from NDC into pixels, while the scissor merely decides what is allowed through.