means The apparent shift in an object's position when you view it from two different vantage points — and the technique that uses that shift to measure distances, especially to stars.

from From Greek 'parallaxis,' meaning alteration or change, built from 'para-' (beside, alongside) and 'allassein' (to change, from 'allos,' other). It entered scientific use through astronomy, where the tiny apparent wobble of nearby stars against the distant background — seen as Earth swings to opposite sides of its orbit — became the first real yardstick for the cosmos.