—n. 1. Geom. a. a solid geometric figure generated by the revolution of a semicircle about its diameter; a round body whose surface is at all points equidistant from the center. Equation: x2 + y2 + z2 = r2.
b. the surface of such a figure; a spherical surface.
2. any rounded body approximately of this form; a globular mass, shell, etc.
3. Astron. a. a planet or star; heavenly body.
b. See celestial sphere. c. any of the transparent, concentric, spherical shells, or layers, in which, according to ancient belief, the planets, stars, and other heavenly bodies were set.
4. the place or environment within which a person or thing exists; a field of activity or operation: to be out of one's professional sphere. 5. a particular social world, stratum of society, or walk of life: His social sphere is small. 6. a field of something specified: a sphere of knowledge.
—v.t. 1. to enclose in or as if in a sphere.
2. to form into a sphere.
3. to place among the heavenly spheres.
a combining form of sphere (planisphere); having a special use in the names of the layers of gases and the like surrounding the earth and other celestial bodies (ionosphere).