In an optical system, the entrance pupil is a virtual aperture that defines the area at the entrance of the system that can accept light. Rays that pass through the pupil are able to enter the optical system and pass through it to the exit (neglecting vignetting).
The entrance pupil is the image (usually virtual) of the aperture stop in the optics that come before it. In a camera, the aperture stop is the diaphragm aperture in the camera that the photographer adjusts to control how much light reaches the film. The setting of the aperture is typically represented by the f-number, which is the ratio of the focal length of the lens to the diameter of the entrance pupil (not the diameter of the actual aperture itself). That is, EN = f/N, where EN is the diameter of the entrance pupil, f is the focal length, and N is the f-number ("relative aperture").
Depending on the lens design, the entrance pupil's location on the optical axis may be behind, within or in front of the lens system, or even at infinity in the case of telecentric systems. The location of the entrance pupil is important in panoramic photography, because the camera must be rotated about the centre of the entrance pupil to prevent parallax error when photographs are stitched together into a panorama. As a result, the centre of the pupil is sometimes called the "no-parallax point" of the lens.