The Collaborative International Dictionary of English v.0.48:

Pneumatics \Pneu*mat"ics\, n. [Cf. F. pneumatique.]
   [1913 Webster]
   1. That branch of science which treats of the mechanical
      properties of air and other elastic fluids, as of their
      weight, pressure, elasticity, etc. See Mechanics.
      [1913 Webster]

   2. (Philos. & Theol.) The scientific study or knowledge of
      spiritual beings and their relations to God, angels, and
      men.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mechanics \Me*chan"ics\, n. [Cf. F. m['e]canique.]
   That science, or branch of applied mathematics, which treats
   of the action of forces on bodies.
   [1913 Webster]

   Note: That part of mechanics which considers the action of
         forces in producing rest or equilibrium is called
         statics; that which relates to such action in
         producing motion is called dynamics. The term
         mechanics includes the action of forces on all bodies,
         whether solid, liquid, or gaseous. It is sometimes,
         however, and formerly was often, used distinctively of
         solid bodies only: The mechanics of liquid bodies is
         called also hydrostatics, or hydrodynamics,
         according as the laws of rest or of motion are
         considered. The mechanics of gaseous bodies is called
         also pneumatics. The mechanics of fluids in motion,
         with special reference to the methods of obtaining from
         them useful results, constitutes hydraulics.
         [1913 Webster]

   Animal mechanics (Physiol.), that portion of physiology
      which has for its object the investigation of the laws of
      equilibrium and motion in the animal body. The most
      important mechanical principle is that of the lever, the
      bones forming the arms of the levers, the contractile
      muscles the power, the joints the fulcra or points of
      support, while the weight of the body or of the individual
      limbs constitutes the weight or resistance.

   Applied mechanics, the principles of abstract mechanics
      applied to human art; also, the practical application of
      the laws of matter and motion to the construction of
      machines and structures of all kinds.

   orbital mechanics, the principles governing the motion of
      bodies in orbit around other bodies under gravitational
      influence, such as artificial Earth satellites.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

pneumatics
    n 1: the branch of mechanics that deals with the mechanical
         properties of gases

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "pneumatics":
   aerial photography, aeroballistics, aerodynamics, aerogeology,
   aerography, aerology, aeromechanics, aeromedicine, aerometry,
   aeronautical meteorology, aerophotography, aerophysics, aeroscopy,
   aerospace research, aerostatics, air, anemology, atmosphere,
   aviation technology, avionics, barometry, climatography,
   climatology, fluid, forecasting, gas, halogen gas, hydrostatics,
   inert gas, kinematics, kinetics, long-range forecasting,
   meteorology, microclimatology, nephology, photometry, rocketry,
   supersonics, weatherology