= image of the physical aperture of the lens , as mapped by the lens elements between aperture and object. For front-aperture-lenses, entrance pupil and front aperture are identical.
This is the apparent opening that we see as humans when we look towards lens from the object side.
Light of _each_ object point in the field of view of the lens reaches _each_ point on the front lens of the objective, but only the light directed to the entrance pupil of the lens reaches the matching image point.
The (object side) chief ray angle is the angle between optical axis of the lens and all chief rays. The viewing angle of the lens is twice max chief ray angle.
Here a 3D Graphics that shows in yellow the light from the Objetpoint that heads the entrance pupil and on its way hit’s the front lens of the system. Move the green point.
The example above is over emphasized. Here a more realistic example with a front iris, which also acts as a entrance pupil: