In classical
geometry, the
radius of a
circle or
sphere is a 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 (from the Latin plural) 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 an object does not have a 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.
