The line segment joining any two points on the circle is called a chord.

A circle is a closed curve in which all points are equidistant from a fixed point called the center. The points on the circle are also equidistant from the center. The line segment that connects any two points on the circumference of a circle is known as a chord.

Properties of a chord:
- A chord is always a straight line segment.
- It lies entirely within the circle.
- It can be of different lengths, depending on the distance between the two points on the circle.
- A chord can be either a diameter or not a diameter.

Difference between chord, radius, diameter, and center:
- Chord: A chord is a line segment that connects any two points on the circumference of a circle.
- Radius: A radius is a line segment that connects the center of the circle to any point on the circumference. It is always half the length of the diameter.
- Diameter: A diameter is a chord that passes through the center of the circle. It is the longest chord in a circle and its length is twice the length of the radius.
- Center: The center of a circle is the fixed point from which all points on the circumference are equidistant.

Importance of chords:
- Chords play a significant role in the geometry of circles.
- They help in determining the length of arcs and angles within a circle.
- Chords are used to construct inscribed polygons within a circle.
- They are also used in the construction of tangents and secants to a circle.

In conclusion, the line segment joining any two points on the circumference of a circle is called a chord. It is a straight line segment that lies entirely within the circle and can have various lengths. Understanding the properties and significance of chords is essential in the study of circle geometry.