The shortest line between two points is NOT always the segment that joins (or jion) them on a plane: the answer depends on the concept of distance or the metric used for the space. If using a taxicab or Manhattan metric it is the sum of the North-South distance and the East-West distance. There are many other possible metrics.
The proof for a general metric is the Cauchy-Schwartz inequality but this site is totally incapable of dealing with the mathematical symbols required to prove it.
A diagonal from a polygon is a line segment that joins two nonconsecutive points.
A line segment that intersects with or joins two points on a circle is called a chord.
That may mean the line segment that joins points labelled Q and Z.
………………………… is a line segment that joins two vertexof a polygon but is not a side of the polygon.
mid-segment
Altitude
the midsegment
Not sure what two non-consecutive points are. No two points are consecutive in the sense that an infinite number of points can be found between any two points.
a midsegment of a triangle
It is the radius of the circle
midsegment
The diagonal joins two non-adjacent vertices in a rectangle.