A Sector is a region of space as identified by the United Nations Space Command. Sectors usually contain several star systems, some of which are UEG colony worlds. Other sectors contain no planets at all.
There appear to be two systems for naming sectors, one for inhabited sectors, one for uninhabited sectors. The UNSC Navy designates sectors as "FLEETCOM Sector (number)" to denote the division of command the sector's inhabitants fall under. Other sectors are simply designated "Sector (letter)-(three numbers)".