It is a Polygon.Polygon
a closed figure with all straight line segments is called a polygon.
Figures and shapes that have no perimeter such as a line segment
I believe it may be called a diagonal.
This describes a closed line segment. We sometimes see it called a line segment or just a segment.A segment is a part of line that contains 2 endpoints and all the points between them.
A line of symmetry divides a figure into two matching halves.
It, too, is called a line segment.
The middle of a line segment is called a Midpoint.;-)
A line segment has two end points. This is called an line segment.