The word radio comes from Latin radĭus ("lightning") and has several meanings:

• In Geometry, the radius is the straight line that can be drawn from the center of a circle to the circumference. In this sense, a spoke is also a rod that connects the center of a wheel with the rim. The word "radius" is also used to refer to the circular extension of land, taking as a reference a point that serves as the center and the length of the radius.
• In Anatomy, the radius is a bone in the human body that, together with the ulna, forms the forearm.
• In chemistry, radium is a chemical element on the periodic table. Its symbol is Ra and its atomic number is 88. It is a shiny white metal with high radioactivity.

The radius is a circle is a segment that connects the center with any point on the circle. All the radii of a circle have the same length and correspond to half the diameter. One way to find the radius of a circle is to take the square root of the division of the area by the number π. The radius in a sphere is a segment that joins the center with any point on its surface. The radius or apothem in a regular polygon corresponds to the radius of the circumscribed circle. In a regular polyhedron, the radius corresponds to the radius of the circumscribed sphere.