The Collaborative International Dictionary of English v.0.48:

Hydrodynamic \Hy`dro*dy*nam"ic\, Hydrodynamical
\Hy`dro*dy*nam"ic*al\, a. [Hydro-, 1 + dynamic, -ical: cf. F.
   hydrodynamique.]
   Pertaining to, or derived from, the dynamical action of water
   of a liquid; of or pertaining to water power.
   [1913 Webster]

   Hydrodynamic friction, friction produced by the viscosity
      of a liquid in motion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydrodynamic
    adj 1: of or relating to hydrodynamics