Radius

A radius is a line segment from a circle’s center to a point on its circumference. Its length is half the diameter.

Circle: A plane curve whose points are all the same distance from a fixed center. A radius is defined by its endpoints on this curve and at its center.
Pi: The constant ratio of a circle’s circumference to its diameter. It gives the circumference formula in terms of radius: 2πr.
Compass (drawing tool): A tool with two legs used to draw circles and transfer distances. Setting the distance between its points fixes the circle’s radius.
Semicircle: One half of a circle, bounded by a diameter and an arc. Its straight boundary is a diameter, not a radius.
Center (geometry): A point equally distant from every point on a circle. One endpoint of every circle radius is the center.
Area of a circle: The area enclosed by a circle, equal to π times the square of its radius. The radius determines the enclosed area through A = πr².
Disk (mathematics): The set of points inside or on a circle in a plane. A radius sets the disk’s maximum distance from its center.
Apothem: The perpendicular distance from a regular polygon’s center to one of its sides. Unlike a radius, it reaches a polygon’s side rather than a circle’s circumference.
Circumference: The boundary length of a circle, or the boundary itself in geometric contexts. The radius ends at a point on this boundary.
Polar coordinate system: A coordinate system that locates points by distance from an origin and direction. Its radial coordinate measures distance from the origin, analogous to a circle’s radius.

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