Hierarchical Z (Hi-Z)
A downscaled pyramid of the depth buffer where each tile remembers its coarsest depth, so whole surfaces can be rejected before they are rasterized.
Alongside the ordinary z-buffer, modern GPUs keep a mip pyramid of depth: each level stores the farthest (or nearest) depth for a small tile of pixels. Before a triangle is finely rasterized it is tested against the coarse level — if the whole tile lies behind what is already there, it is discarded without touching a single pixel.
The same structure drives GPU-side occlusion culling, where object bounding boxes are tested against the Hi-Z pyramid instead of being drawn. The technique has been hardware-supported since the late 1990s and is usually handled by the driver, but breaks when a shader writes its own depth.