Fixed Pattern Noise
Noise that appears in the same places on every frame, caused by small manufacturing differences between photosites.
In depth
Unlike random noise, fixed pattern noise does not average away when frames are stacked — it reinforces. Because it is consistent, it can be characterised once and subtracted, which is exactly what dark frame and flat field calibration do. It is far more visible in long exposures and at high gain.
Examples in practice
Stacking twenty astrophotography frames removes random noise but leaves a faint grid pattern until calibration frames are applied.