Windage is how much right and left you can adjust the reticle. For example, if you see in rifle scope specifications windage is 1.5 m / 100 m, this means that you can adjust the reticle maximal 0.75 m right and maximal 0.75 m left for hits on your target at 100 m. Windage range is usually specified in MRAD (1 mrad is 10 cm / 100 m) or MOA (1 MOA is 2.9 cm / 100 m). Some manufacturers designate MRADs with an acronym MIL.
This is a necessary function for zeroing the rifle scope since with this feature you can eliminate any misalignment with your weapon, and adjust the reticle exactly on the point of impact of your ammunition.
With windage adjustment, we can also compensate wind drift of the bullet from a straight trajectory. Wind drift is caused by the effect that a side wind has on a bullet.
Practically all newer rifle scopes have the possibility to adjust the reticle left or right (windage) and up or down (elevation). This process is known as zeroing. The upper turret on rifle scope is for elevation adjustment of reticle and side turret on rifle scope is for windage adjustment of the reticle. Hunter rifle scopes have the mechanism of both turrets protected with caps which protect turret from water, damage or any other outside impacts. Turrets are in other words rotatable buttons which you can spin in in both directions.
Every single movement made with the turret produces a »click« sound. Usually, 1 click on European rifle scopes moves hit on target for 1 cm at 100 m range (0.1 MRAD / MIL). On American, Japanese and Chinese scopes 1 click moves the hit on the target for ¼ MOA (minute of angle) which is 7 mm at 100 m range. On Benchrest of F-class rifle scopes, where the corrections have to be very small and precise, the clicks are in 1/8 MOA (3,5 mm / 100 m) or 0.05 MRAD (5 mm / 100 m).
Source: Revija Lovec