principal point

Where the camera's optical axis pierces the image sensor -- the sensor's own optical center, not necessarily its pixel-grid center.

A camera's principal point $(c_x, c_y)$ is the pixel coordinate where the optical axis (the straight line through the lens center, perpendicular to the sensor) intersects the sensor plane. It is one of the intrinsic parameters packed into the camera matrix, and in a real camera it is close to but not exactly the sensor's own geometric center.

Examples

A camera's principal point typically sits within a few pixels of the sensor's geometric center, but manufacturing tolerances mean it is rarely exact.