In geometry, a part of a circle comprehended between two radii and the arch; or a mixed triangle, formed by two radii and the arch of a circle.

A mathematical instrument so marked with lines of sines, tangents, secants, chordsas to fit all radii and scales, and useful in finding the proportion between quantities of the same kind. The sector is founded on the fourth proposition of the sixth book of Euclid, where it is proved that similar triangles have their homologous sides proportional.