### Theory:

A circle is a two-dimensional figure with no corners and edges. It is a simple closed curve but not a polygon.
Parts of the circle:
Centre

The fixed point in which the circle is described is called the centre of the circle.

The line segment joining any point on the circle with its centre is the radius of the circle.

Chord:

A line segment joining two points on a circle is called a chord.

Diameter:

The chord of a circle passing through the centre is called the diameter of the circle. It is twice the length of the radius.

Circumference:

The distance around the circle is its circumference.

Arc:

Any portion of the circumference of a circle is called an arc. It is denoted using the symbol "$\stackrel{⌢}{\phantom{\rule{1.029em}{0ex}}}$"

Sector:

The plane surface enclosed between two radii and the circular arc of a circle is called a sector of a circle.

Segment:

The region enclosed by the chord and an arc is called the segment of the circle.

Semicircle:

The diameter of the circle divides into two semi-circles. Thus, the semicircle is half of the circle.

Interior of the Circle:

The area within the boundary of the circle.

Exterior of the Circle:

The area outside the boundary of the circle.