1. The scientific study of the mechanical properties of fluids (gases and liquids) in motion or at rest, including the observation, description, and mathematical computation of the behavior of fluids.
2. The experimental and mathematical-computational study of the mechanical behavior of fluids.

Fluid mechanics includes the transfer of heat and matter resulting from motion of the fluid, and the driving of the fluid motion due to differences in density which may be induced by temperature, as well as the effects due to temperature dependency of the constants of materials; for example, the viscosity.

3. The study of fluids and gases at rest and in motion which can be divided into hydrostatics, the behavior of liquids at rest; hydrodynamics, the behavior of liquids in motion; and aerodynamics, the behavior of gases in motion.

Hydrostatics takes into account the forces exerted by a liquid in all directions, not just the downward gravitational pull; such as, the upward force exerted on a submerged object that causes bouyancy.

Hydrodynamics is the study of fluid flow and fluid friction, or viscosity.

Aerodynamics is the study of the motion of gases which is most often applied to the study of air and the motion of solid bodies in it.

The science concerned with fluids, either at rest or in motion, and dealing with pressures, velocities, and accelerations in the fluid, including fluid deformation and compression or expansion.