The correct answer is: Principal point.
The principal point is the point on the image plane where the optical axis of the camera intersects the image plane. It is the center of the perspective projection of the camera’s entrance pupil. The principal point is usually denoted by the symbol $P$.
The principal point is important because it is used to calculate the radial and tangential magnifications of the image. The radial magnification is the ratio of the distance from a point on the object to the principal point to the distance from the image of that point to the principal point. The tangential magnification is the ratio of the tangent of the angle between a line from the principal point to a point on the object and the tangent of the angle between a line from the principal point to the image of that point.
The principal point is also used to calculate the location of the image of a point on the object. The image of a point on the object is located at a distance from the principal point that is equal to the distance from the object to the principal point times the radial magnification.
The principal point is not always located at the center of the image. It can be located off-center due to the tilt of the camera’s optical axis. The tilt of the optical axis can be caused by the camera being tilted or by the lens being tilted.
The principal point is also not always located at the same position on the image plane for different cameras. It can vary depending on the design of the camera and the type of lens used.
The principal point is an important concept in photography and photogrammetry. It is used to calculate the magnification of the image and the location of the image of a point on the object. The principal point can be located off-center due to the tilt of the camera’s optical axis. The principal point is also not always located at the same position on the image plane for different cameras.