An angle is a figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle. In mathematics, angles are used to measure the size of an angle. In order to measure the size of an angle, it is necessary to define an undefined term, which is used to define an angle.

## Defining an Angle

An angle is a measure of rotation, usually in degrees, representing the amount that one line has been rotated from another. Angles can be measured in either a clockwise or counterclockwise direction. For example, a clockwise angle of 90° is an angle formed by two lines that have been rotated 90° in a clockwise direction.

The size of an angle is typically defined in terms of its measure in degrees. An angle of 0° is a straight line, while an angle of 180° is a straight line that has been rotated 180°.

## Understanding the Undefined Term

The undefined term used to define an angle is called the vertex of the angle. The vertex of an angle is the common endpoint of the two rays that form the angle. The vertex of an angle is typically used as the starting point when measuring the size of an angle.

When measuring an angle, the vertex is usually located at the origin, or the starting point of the measurement. For example, when measuring an angle of 90°, the vertex will be located at the origin of the measurement. The vertex is then used as the starting point for measuring the size of the angle in terms of its measure in degrees.

The undefined term used to define an angle is an important concept in mathematics, as it is necessary to understand the concept in order to accurately measure the size of an angle. Without the undefined term, it would be difficult to measure the size of an angle accurately.

In conclusion, the undefined term used to define an angle is the vertex of the angle. The vertex is the common endpoint of the two rays that form the angle, and is typically used as the starting point when measuring the size of an angle. Understanding the undefined term is an important concept in mathematics, as it is necessary to understand the concept in order to accurately measure the size of an angle.