Hydrodynamics is the dynamics of liquids, i.e. of low compressible fluids. It is part of rheology, a general science of fluids. For a while this entry will have a redirect fluid dynamics.
L. D. Landau, E. M. Lifschitz, Fluid mechanics (Russian original is Hydrodynamics), vol. 6 of the Course of Theoretical Physics, Pergamon Press
Vladimir I. Arnold, Boris A. Khesin, Topological methods in hydrodynamics Applied Mathematical Sciences 125, Springer
wikipedia: fluid dynamics