Circular Arcs and Circles: Definitions and Examples

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Central and Inscribed Angles: Definitions and Examples

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:07 The Sum of All Dots
  • 0:32 Locus
  • 1:52 Circles
  • 3:56 Lesson Summary
Save Save Save

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Log in or Sign up

Speed Speed
Lesson Transcript
Instructor: Jeff Calareso

Jeff teaches high school English, math and other subjects. He has a master's degree in writing and literature.

What is a circle? In this lesson, find out all about the circle and its many parts, including circular arcs and semicircles. Also, discover how a locus works in creating a circle, parallel lines and more.

The Sum of All Dots

Did you ever look at the stars and think 'Yep, I totally see a lion, a ram and a seated woman'? Because other than the Big Dipper, I usually just see a bunch of dots. I want those dots to form more complete pictures. When we talk about a few dots becoming so great in number that an actual line is formed, we're talking about a locus.


A locus is an invasive insect species that destroys crops and can be summoned by Moses. Wait, no, that's 'locusts' with a 't.' Totally different thing.

A locus is a set of points that share a common property.

Locus Property Points

Let's consider a few examples. If we look at all the points, or the locus of points, that are d distance from a line above or below it, we get parallel lines - not just one or two stars, but so many that it's an actual line.

The locus of points that are equidistant from two parallel lines is a line midway between them and parallel to both. It's like a center line on a perfectly straight road. If we take two points and look at the locus of points that are equidistant from those points, we get a line that bisects the line connecting our two points. It's also perpendicular to that line.

Now those were all boring straight lines. What if we get a bit more interesting? Let's say we look at a few points that are all a fixed distance, d, from a point, P. That looks like a constellation. And what if we connect the dots by looking at all of the points, or the locus of points, that are d distance from point P? We get a circle!

Points in a circle
Points in circle


That's really all a circle is. It's just the set of all points that are the same distance from a central point. We call that central point the center. Creative, I know.

But a circle has many other cool parts to know that have more interesting names. First, we call that distance from the center the radius. That can be any line from the center to the edge.

Radius- Center point to edge of circle
Circular Radius

If we go from one edge of our curve to the other, passing through the center, that's a diameter. A diameter, then, is just two connected radius lines. To remember this, think about how the word 'radius' is shorter than the word 'diameter,' so it's the shorter line.

Diameter- Two connected radius lines
Diameter Pic

If we have a square or a rectangle, we call the distance around the edge the perimeter. Circles, though, are special. They get their very own word for perimeter. We call the distance around the circle the circumference. This word comes from the Latin (of course) and means 'to carry around.'

To unlock this lesson you must be a Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use

Become a member and start learning now.
Become a Member  Back
What teachers are saying about
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account