Entrance Pupil
The apparent size and position of the aperture as seen from in front of the lens, which is what the f-number is calculated from.
Formula / rule
D = f / N
Measured in
mm
In depth
The entrance pupil is a virtual image of the physical iris formed by the elements ahead of it, so it is rarely the same size or in the same place as the iris itself. Its position also matters for panoramic photography: rotating the camera about the entrance pupil is what eliminates parallax between frames, though the point is commonly and incorrectly called the nodal point.
Examples in practice
On a 50 mm f/2 lens the entrance pupil measures 25 mm across, even though the physical iris blades may be smaller.