Which of the following angles is defined as a relationship involving three surfaces?

The angle defined as a relationship involving three surfaces is a point angle. In dentistry, a point angle is formed at the intersection of three surfaces of a tooth. For instance, when discussing a tooth's anatomy, the point angle would refer to the junction where the mesial surface, distal surface, and occlusal surface meet.

Understanding this definition highlights the specific geometric relationships that exist in dental anatomy and emphasizes the importance of recognizing these points when considering dental restorations or when assessing the shapes and forms of dental components. Each type of angle has its own unique properties, which is why the context of the question is crucial in differentiating them.