Definition 1 of 2
Plain English Meaning
A slice of the sky that one controller is responsible for. Each controller works their own slice, and aircraft are handed off from one slice to the next as they fly along.
Full Definition
A defined portion of airspace assigned to a specific air traffic controller for the separation and control of aircraft within that volume. Sectors are bounded laterally and vertically and together make up the larger airspace managed by an Air Route Traffic Control Center (ARTCC) or terminal facility.
Context Anchor
Pilots most often hear this in air traffic control discussions, especially when a flight is passed from one controller or frequency to another.