The electromagnetic radiation given off by accelerating charged particles. This is what happens in an ordinary antenna, but the term usually refers to the radiation emitted when a charged particle beam is bent in a particle-accelerator.
This radiation is not only produced in synchrotrons, but in any circular accelerator and is significant only for electrons. In a free electron laser synchrotron radiation is created by wiggler magnets which bends the electron beam in tight turns.
The radiation produced has a very broad spectrum and the peak frequency depends on the electron energy and the turning radius. The peak frequency is typically around UV and soft X-rays.