1. The theory which states that electromagnetic and light waves have identical properties.
2. The theory that light consists of electromagnetic radiation and therefore obeys Maxwell's equations; contrasted with earlier concepts that light was a stream of tiny particles or light was a wave in a medium of ether.

Maxwell's equations consists of the four fundamental equations that describe the behavior of electric and magnetic fields in time and space and the dependence of these fields on the distribution and behavior of electric charges and currents.

These four partial differential equations relate to the electric and magnetic fields to their sources, charge density, and current density.