From AMS Glossary
The study of fluid motion.
"Fluid" here refers ambiguously to liquids and gases. Although "classical" hydrodynamics was primarily concerned with incompressible fluids, the term aerodynamics has been reserved for such a specialized aspect of compressible fluid flow that most of meteorological dynamics is best included under the general heading of "hydrodynamics." W. and J. Bjerknes refer to the hydrodynamics of compressible fluids as physical hydrodynamics.