Sector

A sector is the region enclosed by two radii and the arc between their endpoints, like a slice cut from a round pizza. A segment is the region enclosed by a chord and an arc. To identify a shaded region, look at its straight boundaries: two radii meeting at the centre indicate a sector, while a chord cutting off part of the circle indicates a segment.

A sector is the region enclosed by two radii and the arc between their ends: it resembles a slice of pizza. A segment is the region enclosed by a chord and an arc: unlike a sector, it does not need two radii as its straight boundaries. The same chord can bound a smaller and a larger segment on opposite sides.

Join the ends of an arc to the centre with two radii: the enclosed region is a sector, like a slice cut from a round cake. Join those same endpoints with a chord instead: the region between the chord and the arc is a segment, like a cap cut off the circle. Both are areas inside the circle; an arc is only part of its boundary.