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 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.