The Circle:
A circle is a plane curve consisting of all points that have the same distance from a fixed point called center the common distance of the points on the curve from the center is called the radius. The region bounded by the circle is also often referred to as a circle (as when we speak of the area of a circle), the curve is referred to as the perimeter or the circumference of the circle.
Note: that when the number of sides of a polygon is increased without limit, the sides merge into one line and polygon becomes a circle.
Parts of a Circle:

The radius of the circle is a straight line drawn from the center to the boundary line or the circumference. The plural of the word radius is radii.

The diameter is the line crossing the circle and passing through the center. It is the twice of the length of the radius.

The circumference of a circle is the boundary line or the perimeter of the circle.

The chord is a straight line joining two points on the circumference points of a circle. The diameter is a special kind of the chord passing through the center.

An arc is a part of the circumference between two points or a continuous piece of a circle. The shorter arc between and is called the minor arc . The longer arc between and is called the major arc .

A semicircle is an arc which is half of the circumference.

A tangent is a straight line which touches the circle. It does not cut the circumference. The point at which it touches, is called the point of contact.