http://en.wikipedia.org/wiki/Line_(geometry)
Wikipedia
Line (geometry)
The notion of line or straight line was introduced by ancient mathematicians to represent straight objects with negligible width and depth. Lines are an idealization of such objects. Thus, until seventeenth century, lines were defined like this: "The line is the first species of quantity, which has only one dimension, namely length, without any width nor depth, and is nothing else than the flow or run of the point
A line segment is a part of a line that is bounded by two distinct end points and contains every point on the line between its end points.
