Precise Definition: A zero-dimensional object that can be located in an N-dimensional space
Line
Euclid's Definition: A length which has no breadth. (Note, Euclid differentiates between a line and a straight line)
Easy Definition: A straight figure with no start or end point.
Precise Definition: A straight one-dimensional locus of pointsthat extends infinitely in both directions. (Unlike Euclid, modern definitions consider the word line to imply a straight line as opposed to a curve.)
Line Segment
Euclid's Definition: The line segment, AB, is the collection of points between point A and point B.
Precise Definition: A finite length determined by a straight one-dimensional locus of points that terminate at points A and B.