<h2>Answer:</h2>
The undefined term that is used to define an angle is:
Line.
<h2>Step-by-step explanation:</h2>Angle is formed with the help of two lines which are not parallel to each other and intersect at a point.
( Also, such two lines are known as a ray.
Since a ray is a line with one end point and the angle is formed with the help of 2 rays with the same starting or initial point )
From the diagram we may see how the angle is formed.
Hence, the answer is:
Line