- You might also be looking for Radius (geometry).
Riker's arm showing ulna and radius.
The radius is a bone in the Human skeletal system. Together with the ulna, it forms the bone structur of the forearm.
When solanagen-based lifeforms conducted experiments on Commander William T. Riker, they severed his forearm. When reattaching the arm, they created an offset of 0.2 microns, visible in a scan of the bone structure of his ulna and radius. (TNG: "Schisms")