Magnetohydrodynamics (MHD) is the study of how conducting fluids, such as liquid metals or plasmas, behave in the presence of electric and magnetic fields. It is a macroscopic form of electrodynamics, deriving from fluid dynamics such as magnetic pressure and magnetic viscosity. Its equations often defy exact solution. The most important applications are in magnetohydrodynamic generators and controlled nuclear fusion process. The extremely hot plasma produced by the fusion is contained by strong circulating magnetic fields; various designs are possible, the stability of each being the paramount consideration.
Magnetohydrodynamics also forms an important part of astrophysics since plasma is one of the commonest forms of matter in the universe, occurring in stars, planetary magnetospheres, and interplanetary and interstellar space.