An angle formed by measuring the position of an object from two points at opposite ends of a baseline.
Astronomers measure the parallax of a star using Earth's orbital diameter as a baseline.
The astronomer observes and photographs a star at a six-month interval, when Earth is at opposite ends of an imaginary baseline defined by the width of its orbit around the Sun. Similarly, you can use the distance between your eyes as a baseline. Look at a distant object, say a telephone pole, with one eye closed then the other to see a parallax shift. Astronomers look for a parallax shift in two star images as a way of estimating a star's distance from Earth. The greater the shift with respect to background stars, the closer the star.