A sphere (from Greek σφαίρα - sphaira, "globe, ball,") is a symmetrical geometrical object. In non-mathematical usage, the term is use to refer either to a round ball or to its two-dimensional surface. In mathematics, a sphere is the set of all points in three-dimensional space (R3) which are at distance r from a fixed point of that space, where r is a positive real number called the radius of the sphere. Thus, in three dimensions, a mathematical sphere is considered to be a two-dimensional spherical surface embedded in three-dimensional space, rather than the volume contained within it (which mathematicians would instead describe as a ball). The fixed point is called the center or centre, and is not part of the sphere itself. The special case of r = 1 is called a unit sphere.