Radius
In classical geometry, the
radius of a circle or sphere is the length of a line segment from its center to its perimeter. The name comes from Latin
radius, meaning "ray" but also the spoke of a chariot wheel. The plural of
radius can be either
radii or the conventional English plural
radiuses. The typical abbreviation and mathematic variable name for "radius" is
r. By extension, the diameter
d is defined as twice the radius: If the object does not have an obvious center, the term may refer to its
circumradius, the radius of its circumscribed circle or circumscribed sphere. In either case, the radius may be more than half the diameter, which is usually defined as the maximum distance between any two points of the figure. The inradius of a geometric figure is usually the radius of the largest circle or sphere contained in it. The inner radius of a ring, tube or other hollow object is the radius of its cavity. For regular polygons, the radius is the same as its circumradius. The inradius of a regular polygon is also called apothem.