Parts of the circle

Circle is a word that comes from the Latin “circulus”, which is a diminutive of “circus”, whose meaning was circle, circular and also circuit. This term is defined as a geometric figure consisting of a closed curve line, ellipse, or parabola that is at all times the same distance from the point called the center.


It is a synonym for the word circumference or what in common language is called a circle. However, there is a certain nuance that differentiates one term from another. Since while the circumference is the geometric line, the circle is the geometric place of the points that are in a circumference.

Another sense that the word circle takes, is that of an organization that offers a common space, which can be both physical and symbolic, in which a group of people can come together to share their shared interests or talk about certain issues that concern them or hobbies.

However, the parts that we will describe below refer to the first definition of this concept.


  • 1Parts of the circle
    • 1Circumference
    • 2Center
    • 3Radio
    • 4Diameter
    • 5Rope
    • 6Arc
    • 7Inner point
    • 8Outside point
    • 9Central angle
    • 10Tangent line
    • 11Secant line
    • 12Inscribed angle
    • 13Semi-inscribed angle
    • 14Interior angle
    • 15Exterior angle
    • 16Circular area

Parts of the circle


Already described within the definition. It is the line that borders the circle, that is, the closed curvature that forms it.


In geometry it is named with the letter C. It is the interior point that is invariably equidistant from the circumference, that is, from its perimeter. All points of the circumference are equidistant in the center.


This is the name given to each of the segments or lines of the circle that start from its center and connect it to any point on the circumference. All points of the circle must be of the same length between them. In geometry it is reflected with the letter r.


It is the straight line or segment (D), which joins two points of the circumference, passing in turn through the center (C) of it. It is the largest segment that joins two points on the circumference, and divides the circle into two halves or equal parts. The diameter is twice the radius, or in other words, the radius is half the diameter.


It is a segment, which in geometry can be expressed with the letter K, and which joins any two points on the perimeter of the circle, as long as it does not pass through the center of the circle. Therefore, the rope will always be less than the diameter. However, the diameter is sometimes considered a string itself.



This term describes the area of ​​the perimeter or circumference of the circle (which can be written with the letter “a” in geometry) that is delimited by the two ends of a rope. That is, the arc can be defined as the curvilinear segment of points that are part of the circumference.

Inner point

Also called “I”. It is any point within the circle, being at a distance from the center point smaller than or equal to the radius.

Outer point

By contrast, the outer point refers to any point outside the circle (E) and the circumference. Therefore, it is at a distance from the center greater than the length of the radius (r).

Central angle

The central angle is two rays whose vertex is common and which is between two segments or radii that start at the center of the circle and end at two points on its perimeter. A central angle establishes an arc.

Tangent line

It is a straight line that is drawn on the outside of the circle, having a single common point with its perimeter. The radius is perpendicular to the tangent at what is called the tangent point. It is named with the letter T in the area of ​​geometry.

Secant line

It is a straight line that cuts the perimeter of the circle at two points. Therefore, it is a segment that passes inside this geometric shape and then extends on the outside of it. The letter S is used to name this line in the mathematical field.

Inscribed angle

This term is defined as two straight lines with a common vertex that create two chords, which coincide at the same place on the circumference. This means, it is an angle that creates three points on the circumference. This part of the circle can be named with the beta letter of the Greek alphabet.

Semi-inscribed angle

It is formed by a rope and a tangent line, which come together, forming its vertex at the perimeter of the circle.

Inner angle

Those lines that join inside the circle and create its vertex there.

Outside angle

When the lines that form it meet outside the circle, passing its segments around its perimeter or through its center.

Circular area

It is an area that is created within the circle, which is delimited by two strings.


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