Definitions of sphere:

noun: any spherically shaped artifact
noun: the geographical area in which one nation is very influential
noun: a three-dimensional closed surface such that every point on the surface is equidistant from the center
noun: a solid figure bounded by a spherical surface (including the space it encloses)
noun: a particular environment or walk of life
Example: "His social sphere is limited"
noun: a particular aspect of life or activity
noun: the apparent surface of the imaginary sphere on which celestial bodies appear to be projected