Sector

A sector on the plane is a region within a plane figure bounded by two rays drawn from an interior point of the figure, and an arc of the contour. A sector of a circle (a circular sector) is a figure bounded by two radii and the arc on which they are based. The area $S$ of a sector of a circle is given by $S=lr/2$, where $r$ is the radius and $l$ is the length of the arc.

A sector in space is a part of a solid bounded by a finite surface, the vertex of which is an interior point of the solid, and by the part cut from the surface of the solid. A spherical sector is a solid produced by rotating a sector of a great circle about one of its bounding radii. The volume of a spherical sector is given by $V=2\pi R^2h/3$, where $R$ is the radius of the sphere and $h$ is the projection of the chord spanning the arc of the sector onto the axis of rotation.

Comments

In mathematical models in various areas, for example economics and theoretical physics, the word sector is used to refer to some more or less clearly defined submodel whose constituents "interact more strongly with each other than with other parts of the model" . Thus, e.g., in econometric models one can have a financial sector, an agricultural sector, etc.