1. The process by which electromagnetic radiation is propagated through free space. The propagation takes place at the speed of light (3. 00 x 10⁸ m s⁻¹ in vacuum) by way of joint (orthogonal) oscillations in the electric and magnetic fields. This process is to be distinguished from other forms of energy transfer such as conduction and convection. 2. Propagation of energy by any physical quantity governed by a wave equation. 3. See alpha ray, beta ray.