It is the line that passes through the midpoint of a segment at a 90 degree angle

The line that passes through the midpoint of a segment at a 90-degree angle is a perpendicular bisector
These are lines that pass throgh the center of a line and perpendicular to that line through
Any line that passes through an angle dividing into two equal part is called a bisector.
Since you are bisecting through a 90° angle, hence it's called a Perpendicular Bisector because you are bisecting a perpendicular line.
Learn more on bisectors here: https://brainly.com/question/11006922