<hardware> (Or "Optical Signal Processing") Operating on data represented using electromagnetic radiation, e.g. visible light, instead of the electrical signals used in a conventional electronic digital computer.
Electronic digital computers are built from transistors. These form components that store data and logic gates that perform the low-level Boolean operations such as AND, OR and NOT that are the basis of all digital computation.
The optical equivalent requires material with a non-linear refractive index such that light beams can interact with each other to perform the same Boolean operations.
Though the photons that carry optical signals offer some theoretical advantages over the electrons that carry electronic signals, there are many practical problems that would have to be overcome before optical computing could compete in terms of cost, power and speed.