In physics, radiation is a process in which electromagnetic waves (EMR) travel through a vacuum or through matter-containing media; the existence of a medium to propagate the waves is not required. A different but related definition says radiation is a subset of these electromagnetic waves combined with a class of energetic subatomic particles with very high kinetic energies; these are called ionizing radiation, and the particles are termed particle radiation. Other sorts of waves, such as acoustic, seismic, hydraulic and so on are not usually considered to be forms of "radiation" in either sense. We will consider the first definition, and return to the second later.