The apparent difference in position of an object when seen from two different viewpoints.
The problem with cameras having a viewfinder separate from the main lens (as opposed to SLR reflex cameras) is that the main lens and the viewfinder see the subject from slightly different positions.
This causes framing errors. A partial solution is to tilt the axis of the viewfinder, but a different amount of tilt is required depending on the distance to the subject.
External viewfinders sometimes have a parallax compensation knob, which tilts the viewfinder according to a distance scale. Built-in viewfinders are usually set for correct framing at infinity, but may have parallax compensation marks in the framing window for use at short lens to subject distances. Single-Lens reflex cameras do not suffer from viewfinder parallax.