A circle is a twodimensional plane geometrical figure which is a closed region. A circle can be defined as a set of points adjacent to each other and are equidistant from a fixed point. There is a wide range of examples for the circular objects in and across our surroundings. A few examples of circular objects are:

Bangles used as ornaments

The base of a tumbler

The lower end of the cone

Top and bottom of cylinders
Regions of Circle
The circle has three regions as in every closed geometric figure. A point can lie on any of the three regions of the circle, namely:

Interior of the Circle:
The region inside the boundary of the circle is called its interior and the point lies in this region is called the interior point.

The exterior of the Circle:
The region outside the boundary of the circle is called the exterior of the circle and any point lying outside the circle is said to be in its exterior.

On the Circle:
Any point on the boundary of the circle is said to be on the circle. The point on the circle coincides with any one of the points that constitute the circle.
Parts of a Circle
A circle is an interesting plane geometric figure which has a lot of importance in mathematics. It represents the atom, which is the basic constituent of the universe, and the entire universe is also represented by the circle. Spiritually, the circle is used to represent that everything in the materialistic world is ultimately zero (0). The circle has many parts, which include:

Centre: The centre of the circle is that point from which all the points that constitute a circle are equidistant. It is generally represented by the letter ‘O’.

Circumference: The region enclosed by a circle or the length of the boundary of the circle is called its circumference. The circumference of a circle is the measure of the total length of the circle, which is also known as its perimeter.

Radius: The length of the line joining the centre of the circle and any point on its circumference is called the radius of the circle. It is generally represented by the letter ‘r’.

Diameter: The line joining any two points on the circle passing through its centre is called the diameter of the circle. Diameter is represented by the letter ‘d’. The diameter of a circle is two times its radius.

Chord: Any line that touches the two points on the circle is called the chord of the circle. The diameter of a circle is its longest chord.

Arc: The portion of a circumference of the boundary of the circle is called its arc. The smaller portion of the circle’s boundary is called its minor arc and the larger portion is called the major arc.

Segment: The region of the circle enclosed by an arc and the chord is called the segment of the circle. The chord divides the circle into two segments. The region enclosed by a chord and the major arc is called the major segment and the region enclosed by the minor arc and the chord is called a minor segment.

Semicircle: The diameter divides the circle into two equal segments. Each segment is called the semicircle which is equivalent to half of the circle.

Sector: The region enclosed by two radii and an arc of the circle is called the sector of the circle. Any two radii divide the circle into two sectors. The region enclosed between the radii and the major arc is called the major sector and the region enclosed by the radii and the minor arc is called the minor sector.

Tangent: Tangent is the line that touches the circle at only one point on its boundary is called the tangent of the circle. Any point on the circumference of the circle has only one tangent.

Secant: The line passing through the circle at two different points is called the secant of the circle.

Area of a circle: The region enclosed by the surface of the circle on a plane is called its area.
Important Formulas

Circumference of the circle = 2 x π x radius

Diameter of the circle = 2 x radius

Area of the circle = π x radius x radius = πr2

Area of the sector of the circle is A = [frac{theta}{360} pi r^{2}]

Area of a segment of a circle = Area of the sector – Area of triangle
Did you Know?
The ancient Greeks thought of the circle as the perfect shape. Can you guess why? How many lines of symmetry do you think a circle has? To the ancient Greeks, the circle served as a symbol signifying divine symmetry and balance that could be found in nature. It fascinated the Greek mathematicians to learn about the geometry of circles and they have spent entire centuries exploring their properties.
There is a type of circle that fascinates people around the world even today, which is the crop circle. Recently, they have been a constant subject of conspiracy theories and pranks. However, ancient reports have been discovered as well, that have mentioned these crop circles. It is still not known to anybody how such complicated patterns can be formed in nature.
The parts mentioned in this article are closely related to finding out more about such patterns and their occurrences. These formulas and parts can definitely help further the research on these crop circles, and maybe, we will someday find out what they mean and if they are some sort of signal!
Quiz Time
It is easy to read through an article or a textbook. However, retaining newly gained knowledge and understanding the concepts clearly can be tricky. The best way is to try your hands at this little quiz and see how many of these you get correct.

Are all c
ircles similar?

Yes

No

Are all circles congruent?

Yes

No

How many tangents can be drawn to a circle from a point at its exterior?

One and only one

More than 4

Exactly two

How many secants can be drawn to a circle from a point on its exterior?

None

Two

One

More than 2
Conclusion
To sum it up, in Maths and Geometry, circles are special kinds of ellipses, where the figures have zero eccentricity and two coincidental foci. A circle can also be defined as the locus of the points that are drawn at an equal distance from a centre. The distance between the centre of a circle and its outer line is known as its radius.