\begin{definition}[Definition:Sector]
A '''sector''' of a circle is the area bounded by two radii and an arc.
{{EuclidSaid}}
:''{{:Definition:Euclid's Definitions - Book III/10 - Sector of Circle}}''
{{EuclidDefRefNocat|III|10|Sector of Circle}}
In the diagram below, $BAC$ is a '''sector'''. 
:300px
In fact there are two '''sectors''', together making up the whole of the circle.
When describing a sector by means of its three defining points (as in $BAC$ above), the convention is to report them in the following order:
:$(1):\quad$ The point on the circumference at the end of the clockwise radius
:$(2):\quad$ The point at the center of the circle
:$(3):\quad$ The point on the circumference at the end of the anticlockwise radius
Thus in the '''sector''' above, $BAC$ describes the '''sector''' indicated by $\theta$, while the '''sector''' comprising the rest of the circle is described by $CAB$.
\end{definition}