Definition:Angle Bisector



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 $$\angle ABD + \angle DBC = \angle ABC$$.

Note
The construction of an angle bisector is Proposition 9 of Book I of Euclid's.