In ranged weapons such as firearms and artillery pieces, the act of sighting in or sight-in is a preparatory or corrective calibration of the sights with the goal of having the projectile (e.g. bullet or shell) placed on a predictable impact position in relation to the sight picture. The principle of sighting-in is to shift the line of aim until it intersects the parabolic projectile trajectory at a designated point of reference, so when the gun is fired in the future (provided there is reliable precision) it will repeatably hit where it aims at identical distances of that designated point.
Because when using a telescopic sight, the crosshair lines geometrically resemble the X- and Y-axis of the Cartesian coordinate system where the reticle center is analogous to the origin point (i.e. coordinate [0,0]), the designated sighting-in point is known as a zero, and the act of sighting-in is therefore also called zeroing. A gunsight that remains true to its designated zero after repeated usage is known as to "hold zero", while one that fails to do so is known as to "lose zero".
The iterative procedure involves firing a group of shots from a cool gun barrel, then determining the geometric center of the shot pattern, adjusting the sights to move the point of aim to that group center, and repeating the process until further groups consistently center on the point of aim.