Definition: The part of a circle enclosed by two
radii of a circle and their
intercepted arc.
A pie-shaped part of a circle.

Try this Drag one of the orange dots that define the endpoints of the blue arc.
The sector of the circle is shown in yellow.

As you can see from the figure above, a sector is a pie-shaped part of a circle. It has two straight sides (the two radius lines),
the curved edge defined by the arc, and touches the center of the circle.