Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and shifts toward higher frequencies as the change of the energy of the accelerated particles increases.
Strictly speaking, bremsstrahlung is any radiation due to the acceleration of a charged particle, which includes synchrotron radiation and cyclotron radiation; however, it is frequently used in the more narrow sense of radiation from electrons stopping in matter.
Bremsstrahlung produced by a high-energy electron deflected in the electric field of an atomic nucleusBremsstrahlung emitted from plasma is sometimes referred to as free-free radiation. This refers to the fact that the radiation in this case is created by charged particles that are free both before and after the deflection (acceleration) that causes the emission.
Source: wikipedia, NASA