Parallax
The apparent shift in an object's position when viewed from two different locations, used to measure distance in astronomy and everyday vision.
The apparent displacement of an object against a distant background when observed from two different vantage points. In astronomy, parallax measures stellar distance by comparing a star's position as Earth orbits the sun; in everyday life, it's why your finger appears to jump when you close each eye alternately. The greater the parallax angle, the closer the object.
What this means in real life
Hold your finger at arm's length and close one eye, then the other—your finger seems to jump side to side against the wall behind it. That shift is parallax; it's how your brain judges how far away your finger is.
What it isn’t
Parallax is not the same as perspective or depth of field in photography. It's specifically the *angular shift* in an object's apparent position from two viewpoints, not simply how things look smaller or blurrier at distance.
Commonly misused online
Social media often conflates parallax with the parallax scrolling effect in web design—where background layers move slower than foreground. While inspired by the optical principle, this is a visual trick, not true parallax measurement.