Parallax Error
Aiming error that appears when the target and reticle sit on different focal planes and the eye shifts, removed by setting parallax for the range.
Parallax error is the apparent shift between the reticle and the target that occurs when the two are not on the same focal plane and the eye moves behind the eyepiece. When parallax is uncorrected for the actual distance, any movement of the eye makes the reticle appear to crawl across the target, so two shots aimed identically can land apart. The effect grows with magnification and with the gap between the set range and the true range, which makes it a real source of dispersion at long distance.
The fix is to dial the focus control until the reticle stops moving when the head shifts, which lands the target image on the reticle plane. A side focus turret or an adjustable objective ring lets the shooter tune this for each engagement without leaving position. A reliable cheek weld also helps, since keeping the eye in the same spot every shot minimizes whatever residual error remains.