For some reason, "parallax" seems to be a difficult concept for some folks to understand, although it is really not complex. The American Heritage Dictionary defines parallax as: "An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight." This is a simple effect that we have all experienced.

In a discussion of optical sights, parallax usually refers to the apparent movement of the reticle (or red dot) relative to the target when you move your head, as you look through the scope. For technical reasons, parallax can only be eliminated from an optical sight at one range. This having been done, parallax is usually not apparent in normal use, unless there is something wrong with the sight.

Leupold, for example, typically designs their centerfire rifle hunting scopes to be parallax free at 150 yards. Leupold rimfire rifle and shotgun scopes are designed to be parallax free at 75 yards.

Some telescopic sights, usually long range varmint or target models, come with an adjustable objective that can be adjusted to eliminate parallax at specific distances. These are useful when shooting at known distances, as in target matches.

In his Guns and Shooting Online article Riflescopes, where do I Start? (A Starter Guide to Riflescopes), Thomas Macarle, who is the CEO of Mac Arms, explained it this way:

There is another way the word parallax applies to guns and sights. The line of sight is different from the axis of the bore, because the sight is mounted above the bore. This distance between the line of sight and the center of the bore (about 1.5 inches for a low mounted scope) creates parallax (see definition above). If you look through the bore, your observational position is slightly different from when you look through the riflescope. To minimize the effect of this parallax, always mount an optical sight as close to the bore as possible.