Is there a term for where a camera’s optical axis intersects the ground in a scene?

There doesn’t appear to be a standard photography term for that point in the scene itself. In imaging/engineering, the related term on the image side is the principal point, which is the optical center on the image plane where that scene point would project.

It’s not the focal plane or image plane: those refer to planes associated with focus or the sensor/film, not a specific point out in the scene. Also, unless the optical axis is exactly perpendicular to the ground, the ground is at varying distances from the camera, so the scene point where the axis meets the ground is just a geometric intersection rather than a commonly named photographic feature.

So the clearest description is simply “the point where the optical axis intersects the ground plane” (or scene plane).