A parallax error is the perceived shift in an object's position as it is viewed from different angles. The error is most easily noticed by looking at a nearby object with one eye closed, then looking at it through the other eye. The apparent motion of the object is the parallax shift, and it is responsible for a small, but noticeable, error common to optical equipment.
The parallax error is commonly encountered when looking downrange through a telescopic gunsight. The scope is mounted somewhat above the barrel, through which the bullet is fired, and is therefore slightly misaligned. Sighting a target through the scope puts the target in line only with the scope. The difference in position between the sight's objective lens and the muzzle of the rifle introduces a parallax error that must be corrected for in order to place the round on the target.
In astronomy, the parallax error is used to establish the distance to nearby stars. The apparent shift in stars' positions, against the "fixed" background of more distant stars, is a function of the stars' distance from Earth. In this case, the parallax baseline is the distance from one point in Earth's orbit to another.