Definition:Line/Straight Line Segment

Definition
A straight line segment is a line segment which is straight.


 * A straight line segment can be drawn joining any two points.

(: Postulates: Euclid's Second Postulate)

Thus a definition for straight line which is frequently encountered is:
 * A straight line is the shortest distance between two points.

This is all very well but it assumes that the line in question terminates at two particular endpoints.