Totality of space points which are equidistant at R from a given point O is called a sphere. Totality of space points which are of a distance not greater than R from a given point O is called a ball. A ball is written as a sphere: w(O,R). The point O is called the centre of a sphere (of a ball). A segment joining the centre of a sphere to any its point is called the radius of the sphere (of the ball). A segment joining two any sphere points is called a chord of the sphere (of the ball). Sometimes the length is meant by the radius or the chord. A chord passing through the centre of a sphere is called its diameter. A line drawn through a sphere point perpendicular to the radius at the point is called a sphere tangential line.
