The aperture through which light enters an optical system; the image of the aperture stop as viewed from the front; specifically the image of pupil of the eye as seen from outside the eye; the pupil itself.
Origin
Late 19th century.
Definition of entrance pupil in US English:
entrance pupil
noun
The aperture through which light enters an optical system; the image of the aperture stop as viewed from the front; specifically the image of pupil of the eye as seen from outside the eye; the pupil itself.