magnetohydrodynamics (MHD), the description of the behaviour of a plasma (q.v.), or, in general, any electrically conducting fluid in the presence of electric and magnetic fields.
A plasma can be defined in terms of its constituents, using equations to describe the behaviour of the electrons, ions, neutral particles, etc. It is often more convenient, however, to treat it as a single fluid, even though it differs from fluids that are not ionized in that it is strongly influenced by electric and magnetic fields, both of which can be imposed on the plasma or generated by the plasma; the equations describing the behaviour of the plasma, therefore, must involve the close relationship between the plasma and the associated fields.
The inclusion of magnetic effects gives rise to a number of quantities that have counterparts in ordinary fluid mechanics—for example, magnetic viscosity, pressure, Reynolds number, and diffusion.