Line of Sight
Also: LOS
The straight line from the shooter's eye through the scope reticle to the target. The bullet's path crosses this line twice on most zeros.
The bore of the rifle sits below the scope’s optical axis, usually 1.5 to 2.5 inches lower. With a 100-yard zero, the bullet’s curving trajectory crosses the line of sight just past the muzzle (about 25 yards out), arcs above it, then comes back down to cross again at 100 yards.
Past the zero, the bullet falls below the line of sight. That gap is what the shooter dials or holds to correct.