What undefined term is needed to define an angle?

We can define an angle using the undefined term of a line.

Undefined Terms in Geometry

In geometry, we have three terms that are coined as undefined terms. Those are points, lines, and planes. These are called undefined terms, because we cannot define them using any other geometrical term.

The best we can do for a definition of these terms is to name them, but we do not have formal definitions for these three terms.

However, these three undefined terms are the terms that we use to define all of the other terms used in geometry, so they are extremely important in this branch of mathematics.