The beam of light your eye receives: aperture ÷ magnification, or eyepiece ÷ focal ratio. Both routes, with guidance on the useful 0.5-7 mm range.
Exit pupil = aperture ÷ magnification, or equivalently eyepiece focal length ÷ focal ratio. A 150 mm scope at 120x gives 1.25 mm. Above ~7 mm (dark-adapted eye) light is wasted; below ~0.5 mm the view goes dim and floaters intrude. 2-5 mm is the comfortable sweet spot for most targets.
The diameter of the light beam leaving the eyepiece and entering your eye: aperture ÷ magnification. A 150 mm telescope at 120x delivers a 1.25 mm exit pupil; 10x50 binoculars deliver 5 mm.
It sets how bright extended objects look and whether all the collected light fits into your pupil. Bigger than ~7 mm wastes light (your pupil cannot open that far); smaller than ~0.5 mm looks dim and shows retinal floaters.
Exit pupil also equals eyepiece focal length ÷ telescope focal ratio: a 10 mm eyepiece in an f/8 scope always gives 1.25 mm, whatever the aperture. Handy for choosing eyepieces for a given scope.