Holdunder
Aiming below the center of the reticle for very close shots, where the bullet still travels above the line of sight on its way to the target.
Holdunder is the opposite of holdover: the shooter aims below the center of the reticle because at very close range the bullet has not yet risen to cross the line of sight. Since the bore sits beneath the optic, a bullet leaves the muzzle below the aiming line, climbs up through it at the near crossing of the zero, and only then begins its long descent. Inside that near crossing the bullet prints high, so the shooter must hold under the intended impact.
This correction matters for close targets and for rifles zeroed at longer distances, where the bullet can sit several inches above the line of sight at the midpoint of its arc. Within a cartridge’s point blank range the error stays small enough to hold dead center on a large target, but for precise close shots, holding under keeps the bullet from striking high.