  Hydrodynamics is a theory of long-range excitations controlled by equations
of motion that encode the conservation of a set of currents (energy, momentum,
charge, etc.) associated with explicitly realized global symmetries. If a
system possesses additional weakly broken symmetries, the low-energy
hydrodynamic degrees of freedom also couple to a few other "approximately
conserved" quantities with parametrically long relaxation times. It is often
useful to consider such approximately conserved operators and corresponding new
massive modes within the low-energy effective theory, which we refer to as
quasihydrodynamics. Examples of quasihydrodynamics are numerous, with the most
transparent among them hydrodynamics with weakly broken translational symmetry.
Here, we show how a number of other theories, normally not thought of in this
context, can also be understood within a broader framework of
quasihydrodynamics: in particular, the M\"uller-Israel-Stewart theory and
magnetohydrodynamics coupled to dynamical electric fields. While historical
formulations of quasihydrodynamic theories were typically highly
phenomenological, here, we develop a holographic formalism to systematically
derive such theories from a (microscopic) dual gravitational description.
Beyond laying out a general holographic algorithm, we show how the
M\"uller-Israel-Stewart theory can be understood from a dual higher-derivative
gravity theory and magnetohydrodynamics from a dual theory with two-form bulk
fields. In the latter example, this allows us to unambiguously demonstrate the
existence of dynamical photons in the holographic description of
magnetohydrodynamics.
