Definition
Radius
A radius reaches from a circle’s centre to any point on the circle.
What it means
Start at the exact centre and travel straight to the edge. Any direction gives the same distance, so every radius in one circle has equal length. A diameter crosses the whole circle through the centre, making it two radii end to end.
Remember it
Centre to edge is one radius; edge to edge through the centre is two.
Example
A circle has diameter 18 cm. Since a diameter is two radii, its radius is 18 ÷ 2 = 9 cm.
Watch for
A segment joining two edge points is not necessarily a radius; one endpoint must be the centre.
Common mix-up
Radius and diameter are not the same length. The diameter is twice the radius.
Exact meaning
A radius of a circle is a line segment from the centre to a point on the circle; radius also names the segment’s length. All radii of one circle are equal, and the radius is half the diameter. In a sphere, it runs from the centre to the surface.