Physics: Lateral Displacement?

I presume your term "lateral displacement" is used in optics.
Just as the name implies, "lateral" means "sideways" in English. Thus, "lateral displacement" means the "sideward movement"of a beam of light.
Example, when a light ray incident obliquely onto a glass block, the emergenct ray will go out from the block in the same direction as the incident ray, but is displaced sideway by a certain distance.