Definition:Line/Straight Line

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In the words of Euclid:

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

(The Elements: Book $\text{I}$: Definition $4$)

Also defined as

A definition for straight line which is frequently encountered in the popular literature 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.

However, this is a perfectly valid definition of the concept of a straight line segment.

Also see

  • Results about straight lines can be found here.