Parallax is the difference in the apparent position of an object from one point of perspective to another, based on the location of the object’s viewer. The parallax is measured as the angle between those two points of perspective.

Essentially this is the term to describe the reason why, when moving from one side to another, objects closer to the viewer appear to be moving both faster and a greater distance across the plain of view (say the view finder in your camera) than objects which are at a greater distance. The closer object has a greater parallax, thus the sense that a greater two-dimensional distance is by traveled it. This is illustrated in the short video below. The blue square, which is closer to the camera, moves across the entirety of the screen in same span of time, and within the same amount of camera movement from left to right.



This is the reason that the image you shoot with a twin lens reflex camera (TLR) is not the same as the image captured. The actual image captured is always just a little bit different. Two different lenses cannot reflect to same image, no matter how close they are together.

Parallax is also the underlying principle behind stereopsis, which allows one to perceive depth. Think of your eyes as two lenses. Close one, then the other. The things you’re looking at shift in position. That shift, when seen by your two eyes at the same time, creates a measureable sense of depth. This is also the basis of stereoscopic photography, which is marketed as 3D in movie theatres.