Parallax
An optical error where the reticle appears to shift against the target as you move your eye — corrected with a parallax adjustment on higher-power scopes.
Parallax occurs when the target's image and the reticle aren't on the same focal plane inside the optic. The symptom is that the reticle seems to drift across the target as you move your head behind the scope, which can throw off precise shots. The error grows with magnification and at distances different from where the optic is set.
Most red dots and low-power optics are effectively parallax-free for practical use. Higher-magnification rifle scopes include a parallax (or 'side focus') adjustment to match the optic to your target distance, sharpening the image and eliminating the shift. Setting parallax correctly is part of squeezing maximum precision out of a long-range setup.