Bremsstrahlung (braking radiation) is electromagnetic radiation produced when a charged particle decelerates, when its course is changed by another charged particle. A photon is emitted that balances the energy equation. The resulting radiation emission (from a gas/plasma) has a continuous spectrum (continuum emission), as opposed, for example, to emission lines. It is also called free-free radiation, expressing the point that its production does not involve an electron bound, before or afterward, to an atom.
An astronomical source of bremsstrahlung is the intracluster medium associated with a galaxy clusters.
The reverse of bremsstrahlung radiation is also possible, called free-free absorption, where an electron and another particle interact and a photon is absorbed. Free-free absorption occurs in HII regions, affecting their spectrum.