Zeroing any firearm is the process of understanding the relationship of Point Of Aim (where the shooter aims the firearm) [POA] to Point Of Impact (where the round actually strikes the target) [POI].
For Soldiers to achieve a high level of accuracy and precision, it is critical they zero their [sighting system] to their weapon correctly. The Soldier must first achieve a consistent grouping of a series of shots, then align the mean point of impact of that grouping to the appropriate point of aim.
–Appendix E – Zeroing, Department of the Army Training Circular 3-22.9 – Rifle and Carbine, May 2016
This is the process most shooters are familiar with regarding zero. However, zeroing a fixed sighted handgun is different than zeroing a rifle.
Bottom Line up front: With rifles, we zero the sights to the ammunition. With fixed sighted handguns, we zero (adjust) the ammunition to the sights.