The apparent shift of a nearby star when measured against more distant background stars. The position of the star is measured at 6-month intervals when the Earth is located on opposite sides of its orbit. The resulting small angular shift can be used to calculate its distance.
A star with a parallax angle of one second of arc (1/3600 of a degree) is located one parsec or 3.26 light-years from Earth. In reality no star (apart from the Sun) is so close. Alpha Centauri the nearest star system has a parallax of 0.75 arc seconds, which is equivalent to a distance of 4.3 light-years.