A compression artifact is any visible structure in a picture that the camera did not record and the director did not intend - a block, a stripe, a halo, a smear, or a stutter that lossy encoding or delivery added. Lossy compression discards information the encoder bets you will not miss; push it too hard and the discarded data leaves a visible scar. These artifacts are not random noise: each has a specific mechanism, appears in a specific place, and looks a specific way, which is what makes a field guide possible. They sort into four families by origin - block-transform (blocking, ringing, mosquito noise, blur), tonal (banding), color (color bleeding), and temporal or delivery (judder, stutter, freezing, tiling). The uncomfortable catch is that the popular metrics PSNR, SSIM, and VMAF are blind to several of them, especially banding and the entire temporal family, so a high score never guarantees a clean picture.
