Z-prepass
Rendering the whole scene with depth only and no color first, so the second pass shades each pixel exactly once.
Heavy fragment shaders are expensive to run for nothing when pixels are overwritten (see overdraw). A z-prepass draws the geometry first with no color output, filling only the z-buffer. In the color pass the depth test is set to "equal", so only the frontmost surface per pixel passes and the shader runs a single time.
The cost is submitting the geometry twice, so it pays off when shading is expensive and geometry is cheap — typically in forward rendering with many lights. Deferred shading solves the same over-shading problem differently and rarely needs a prepass.