CK-12 Foundation
Ck 12: Trigonometry: Directed Line Segments
[Free Registration/Login may be required to access all resource tools.] Define a vector mathematically.
CK-12 Foundation
Ck 12: Vectors as Directed Line Segments
[Free Registration/Login may be required to access all resource tools.] Resources for learning about vectors as directed line segments.
Wolfram Research
Wolfram Math World: Line Segment
This site contains a definition of a line segment. Includes a graphic and links to related sites.