Definition
CONTROL SECTOR
An airspace area of defined horizontal and vertical dimensions for which a controller or group of controllers has air traffic control responsibility, normally within an air route traffic control center or an approach control facility. Sectors are established based on predominant traffic flows, altitude strata, and controller workload. Pilot communications during operations within a sector are normally maintained on discrete frequencies assigned to the sector.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).