Naming the parts of a circle
The definition of a circle is that it is a set of points equidistant from its centre
Circumference – the distance around the outside of the circle
Radius – distance from the centre of the circle to the circumference
Diameter – the distance of the width of the circle though the centre.
Arcs and sectors
Arc – is the length which is part of a circumference. Arcs which are shorter than a semicircle is a minor arc and one which is longer than a semi circle is a major arc.
Sector – a part of a circle which is between two radii, an arc and from the centre.