means A perfectly round three-dimensional object in which every point on the surface sits at an equal distance from the center; by extension, a domain or realm of activity or influence.

from From Greek 'sphaira,' a ball or globe — the round thing you tossed in a game. It rolled into Latin as 'sphaera,' then into Old French and on into English. The astronomers gave it grandeur: ancient and medieval thinkers imagined the heavens as nested crystalline 'spheres' carrying the stars, and the music those spheres supposedly made gave us 'the harmony of the spheres.' From those celestial shells we borrowed the figurative sense — your 'sphere' of work or influence is the orbit you move within.