1.
Mathematics. A three-dimensional surface, all points of which are equidistant from a fixed point.
2. A spherical object or figure.
3. A celestial body, such as a planet or star.
4. The sky, appearing as a hemisphere to an observer: the sphere of the heavens.
5. Any of a series of concentric, transparent, revolving globes that together were once thought to contain the moon, sun, planets, and stars.
6. The extent of a person's knowledge, interests, or social position.
7. An area of power, control, or influence; domain. See synonyms at field.