Line is a mark made by a pointed tool - brush, pencil, stick, pen, etc. - and is often defined as a moving dot.
It has length and width, but its width is very thin compared to its length.
A line is created by the movement of a tool and pigment, and often suggests movement in a drawing or painting.
Line in Fine Art
Tobey's painting is all line. He actually drew with his brush, then repeated lines creating a complex pattern. Tobey's lines are the subject of the painting and are not used to outline shapes or objects.
Source: Elements and Principles of Design: Student Guide with Activities, published by Crystal Productions