Line
Definition: A geometrical object that is straight, infinitely long and infinitely thin.

The line goes off in both directions forever, and is perfectly straight. A line, strictly speaking, has no ends and zero width.

If you draw a line with a pencil, examination with a microscope would show that it has a measurable width. In geometry however, a line has no width. The pencil line is just a way to illustrate the idea on paper.