Banding
The visible steps in a smooth gradient — the sky in a video looking as though it were painted in stripes.
The cause is insufficient colour depth. With eight bits per channel there are 256 levels, and a large even area running from dark blue to light blue often needs more intermediate steps than that. Where two adjacent levels meet a visible edge appears, and the eye is uncomfortably good at spotting it.
Heavy compression makes it worse, since the codec merges nearby values to save bits. The remedies are more bits — ten per channel is in practice always enough — or deliberately adding a little noise, which breaks the edge up. The latter is why light film grain sometimes makes footage look better despite containing less information.