Input devices are used to send data or instructions to a computer.
A trackball is an input device because it is used to move the pointer on the screen.
A light pen is also an input device because it is used to select or draw on the screen.
A scanner is an input device because it converts printed text or images into digital form.
A plotter, however, is an output device.
It is used to print large drawings, graphs, maps, engineering designs, and architectural plans.
Therefore, among the given options, the device which is not an input device is:
\[
\text{Plotter}.
\]