Wiktionary
alt. (context geometry English) the portion of a circle enclosed by two radius and an arc n. (context geometry English) the portion of a circle enclosed by two radius and an arc
Wikipedia
A circular sector or circle sector (symbol: ⌔), is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. In the diagram, θ is the central angle in radians, r the radius of the circle, and L is the arc length of the minor sector.
A sector with the central angle of 180° is called a half-disk and is bounded by a diameter and a semicircle. Sectors with other central angles are sometimes given special names, these include quadrants (90°), sextants (60°) and octants (45°), which come from the sector being one 4th or 6th or 8th part of a full circle, respectively.
The angle formed by connecting the endpoints of the arc to any point on the circumference that is not in the sector is equal to half the central angle.