What does parallax mean regarding scopes?
Thanks in advance.

When you move your eye behind the scope, the target may appear to move slightly depending on several factors.  One of these factors is at what distance is the scope is focused vs. the actual distance of the target.  When the target is at the focus distance this movement (called [url=http://digital.library.pitt.edu/parallax/whatisparallax.html]Parallax[/url]) is the lowest (maybe zero) due to nearly parallel path of light exiting a scope to the shooters eye.  Thus one possible advantage of adjustable objective scopes is the focus distance can be set to the target distance.  Typical sporting rifle scopes are adjusted to focus (and thus to be "Parallax Free") at 150 yards, while shotgun / muzzle loader scopes are set at a closer distance.

You can see this effect by holding your finger up watching how the alignment changes compared to the far side of the room when you move your head to the side.  The nearly parallel path of light exiting a scope to the shooters eye greatly reduces the effect, with some of the new optics such as the Aimpoint M2 reducing / eliminating the apparent movement and are "Parallax Free".