A circle is a flat, round shape. Every point on the curved edge is exactly the same distance from the center.
The circle is all the space inside the curved line. In the image below, the circle is the blue filled-in region.
The radius is a straight line from the center of the circle to any point on the edge.
The diameter is a straight line that passes through the center and connects two points on the edge. It is exactly twice the radius, and it cuts the circle into two equal halves.
π (said "Pi") is a special number that tells you how many times the diameter fits around the outside edge of the circle.
π is an irrational number — it goes on forever without repeating. Its approximate value is:
π ≈ 3.141592653589793238…
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