The Collaborative International Dictionary of English v.0.48:

Fluid \Flu"id\ (fl[=u]"[i^]d), a. [L. fluidus, fr. fluere to
   flow: cf. F. fluide. See Fluent.]
   Having particles which easily move and change their relative
   position without a separation of the mass, and which easily
   yield to pressure; capable of flowing; liquid or gaseous.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fluid \Flu"id\, n.
   A fluid substance; a body whose particles move easily among
   themselves.
   [1913 Webster]

   Note: Fluid is a generic term, including liquids and gases as
         species. Water, air, and steam are fluids. By analogy,
         the term was sometimes applied to electricity and
         magnetism, as in phrases electric fluid, magnetic
         fluid, though not strictly appropriate; such usage has
         disappeared.
         [1913 Webster +PJC]

   Fluid dram, or Fluid drachm, a measure of capacity equal
      to one eighth of a fluid ounce.

   Fluid ounce.
   (a) In the United States, a measure of capacity, in
       apothecaries' or wine measure, equal to one sixteenth of
       a pint or 29.57 cubic centimeters. This, for water, is
       about 1.04158 ounces avoirdupois, or 455.6 grains.
   (b) In England, a measure of capacity equal to the twentieth
       part of an imperial pint. For water, this is the weight
       of the avoirdupois ounce, or 437.5 grains.

   Fluids of the body. (Physiol.) The circulating blood and
      lymph, the chyle, the gastric, pancreatic, and intestinal
      juices, the saliva, bile, urine, aqueous humor, and muscle
      serum are the more important fluids of the body. The
      tissues themselves contain a large amount of combined
      water, so much, that an entire human body dried in vacuo
      with a very moderate degree of heat gives about 66 per
      cent of water.

   Burning fluid, Elastic fluid, Electric fluid, Magnetic
   fluid, etc. See under Burning, Elastic, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fluid
    adj 1: subject to change; variable; "a fluid situation fraught
           with uncertainty"; "everything was unstable following the
           coup" [syn: fluid, unstable]
    2: characteristic of a fluid; capable of flowing and easily
       changing shape [syn: fluid, runny]
    3: smooth and unconstrained in movement; "a long, smooth
       stride"; "the fluid motion of a cat"; "the liquid grace of a
       ballerina" [syn: fluent, fluid, liquid, smooth]
    4: in cash or easily convertible to cash; "liquid (or fluid)
       assets" [syn: fluid, liquid]
    5: affording change (especially in social status); "Britain is
       not a truly fluid society"; "upwardly mobile" [syn: fluid,
       mobile]
    n 1: a substance that is fluid at room temperature and pressure
    2: continuous amorphous matter that tends to flow and to conform
       to the outline of its container: a liquid or a gas

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "fluid":
   able to adapt, adaptable, adjustable, aerodynamics, afterdamp, air,
   alterable, alterative, aqueous, atmosphere, beverage, blackdamp,
   blood, breath, changeable, changeful, checkered, chokedamp, cloud,
   damp, drink, effluvium, ever-changing, exhalation, fetid air,
   firedamp, flatus, flexible, flowing, fluent, fluid extract,
   fluid mechanics, fluidal, fluidic, fluxible, fluxile, fluxional,
   fluxionary, formless, fume, gas, halogen gas, hydraulics,
   hydrogeology, ichor, impermanent, indefinite, inert gas, juice,
   juicy, kaleidoscopic, latex, liquid, liquid extract, liquidy,
   liquor, malaria, malleable, many-sided, mephitis, mercurial,
   metamorphic, miasma, milk, mobile, modifiable, movable, mutable,
   nonuniform, permutable, plastic, pliant, pneumatics, protean,
   proteiform, puff of smoke, reek, resilient, rubbery, running,
   runny, sap, sappy, semiliquid, shifting, smoke, smudge, solution,
   steam, succulent, supple, transient, transitory, uncertain,
   unfixed, unformed, unsettled, unstable, unsteady, vapor, variable,
   volatile, water, water vapor, watery, whey