A sector is a part of a whole. As a well the specific usages below, it can also be used metaphorically, referring to a generic piece or portion of something.

• In mathematics (geometry), a sector is the "pie shaped" area of part of a circle enclosed by two radii and an arc.
• In the context of computer hardware, a sector is a sub-division of a track of a magnetic hard disk or optical disc. A sector stores a fixed amount of data. The typical formatting of sectors allows holding 512 bytes (e.g. harddisks and diskettes) or 2048 bytes (e.g. optical discs) of data. (See also hard disk drive partitioning, FAT.)
• In economics, economic activity is often categorised into sectors, in a variety of schemes. For example private sector vs public sector; service sector vs manufacturing sector; primary sector/ secondary sector/ tertiary sector.