1. A line from the center of a circle to a point on the circle.
2. The distance from the center of a circle to a point on the circle.
Try this Drag the orange dot. The blue line will always remain a radius of the circle.
The radius of a circle is the length of the line from the center to any point on its edge. The plural form is radii (pronounced "ray-dee-eye").
In the figure above, drag the orange dot around and see that the radius is always constant at any point on the circle.
Sometimes the word 'radius' is used to refer to the line itself. In that sense you may see "draw a radius of the circle".
In the more recent sense, it is the length of the line, and so is referred to as "the radius of the circle is 1.7 centimeters"
If you know the diameter
Given the diameter of a circle, the radius is simply half the diameter: