Fill in the blank. a bisector is a ray that divides an angle into two angles of measure.

By definition we know that:

An "Angle bisector" is a ray that divides an angle into "two congruent (or equal) adjacent angles".

Therefore, in our question where we have to Fill in the blank, we will use this definition of the "Angle bisector". The answer will be:

A bisector is a ray that divides an angle into two angles of equal measure.