"instrument for drawing objects without the need for artistic skill," 1847; see dia- + -graph.
It consists of a carriage for a pencil governed by a system of cords and pulleys working at right angles to one another, and set in motion by the movement of a pointer, which is passed by the operator, who is careful to keep his eye at a fixed point of view, around the apparent outlines of his subject. The pencil describes on the paper the exact motions of the pointer, and thus reproduces the desired object. [Century Dictionary]
Earlier it meant "a description" (by 1798).