It's an element separated from the surface of the clock. It is usually a parallel bar to the axis of the earth, but this can vary for certain types of sundials, as the ring one that uses the solar light that goes through a hole instead of tip and it impacts in an area in shade, marking the hour.

When it's parallel to the axis of terrestrial rotation, we obtain in the intersection with the plane of the quadrant, the point called POLE. For the pole they pass all the hour lines of true solar time.

The plane that form the tip of the gnomon, the POLE and the perpendicular one from the tip to the plane of the clock, is denominated plane of the gnomon.