The undefined term used to define an angle is ray.

Quick Scoop

An angle in geometry is formed by two rays that share a common endpoint, called the vertex. Each ray starts at the vertex and extends infinitely in one direction, so the concept of a ray is essential for the formal definition of an angle. Because of this, when asked among the options line, plane, point, and ray, the correct choice is ray.

In textbook multiple‑choice questions with these exact options, “ray” is consistently given as the correct answer because an angle is defined as two rays with a common endpoint.

Final answer: ray.