Compression Artefact
Visible distortion introduced by lossy compression, such as blocking, ringing or colour smearing.
In depth
JPEG works on eight-by-eight pixel blocks, so at low quality the block boundaries become visible in smooth gradients, and high-contrast edges develop a faint ringing halo. Chroma subsampling adds colour bleeding along saturated edges. All of these are permanent, and further editing amplifies rather than hides them.
Examples in practice
A heavily compressed sky shows a faint grid of eight-pixel squares once the contrast is increased.