Fragment

A fragment is a rendering concept which is roughly analogous to a single pixel on the output display (as opposed to a texel on an input texture). In practice, there is not a 1:1 mapping between fragments and device pixels for a large variety of reasons, the most obvious of which is anti-aliasing (which introduces additional fragments for some pixels).

When rendering a given primitive, the fragment shader is run once per fragment hit (with the possible exception of fragments that are early z-culled).

When a fragment is hit by multiple overlapping primitives, overdraw occurs. This is considered a waste of GPU performance and should be minimized.