Electromagnetic radiation
According to classical physics, electromagnetic radiation is nothing but a flow of energy through a space or material medium in a form of electric field and magnetic field all together.
The common electromagnetic wave example is radio wave, gama rays and visible lights that flows linking electric field and magnetic field where both two fields are perpendicular in direction.
According to quantum theory, electromagnetic radiation is flow of photon through space. Photon is defined in physics as a pack of energy denoted as hv, where h is called plank constant and v is velosity, that flows with universal speed of light.