segment

A straight line is a line which lies evenly with the points on itself.

The extremities of a line are points.

—Euclid

Translations of Euclid are confusing when referencing lines. We have lines, straight lines, and finite straight lines.

Part of the confusion is that a line is not a thing. It is a relationship projected into a field.

Lines in our system have no end. They are always straight - expressed by a rule. Relationships that are not straight will be called something else, like a curve or arc.

We will define a finite straight line as a segment .

Two points which lie on a line or define a line, can be considered the bounds of a segment of that line.

The values of the points that bound a segment can be used to determine the length of segment.