\begin{definition}[Definition:Angle Bisector]
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Let $\angle ABC$ be an angle.
The '''angle bisector''' of $\angle ABC$ is the straight line which bisects $\angle ABC$.
In the above diagram, $BD$ is the '''angle bisector''' of $\angle ABC$.
Thus $\angle ABD \cong \angle DBC$ and $\angle ABD + \angle DBC = \angle ABC$.
\end{definition}