The radius is defined as the distance from the center to any point on the circumference (outer edge) of a circle or sphere. It is half the diameter.
Radius cheat sheet for homework
Diameter: The distance across a circle or sphere passing through its center, equal to twice the radius.
Circumference: The distance around the outer edge of a circle.
Center: The point inside a circle or sphere that is equidistant from all points on its circumference.