In the field of astrophysics, the term hydrodynamics is commonly used for fluid dynamics, the study of forces and motion in bodies of liquids or gases. In some scientific/engineering fields, the term is used more specifically to refer to the dynamics of non-compressible fluids (liquids), reserving the term aerodynamics for the dynamics of gases. (The astrophysical usage possibly resulted from instances where the formulae for non-compressible fluids was used as a sufficient approximation of the dynamics of gases in some astrophysical situations.)
Magnetohydrodynamics is the study of electrically conducting fluids (either liquid or gas) in motion in an electric field.
A simulations of fluid flows (hydrodynamic model or hydrodynamical model) can be used for modeling Earth oceans or atmosphere, or the same for atmospheres of astronomical bodies.
Other subclasses of hydrodynamics, significant in some extreme situations (e.g., modeling, supernovae).