imponderable

imponderable
    adj 1: difficult or impossible to evaluate with precision; "such
           imponderable human factors as aesthetic sensibility"
           [ant: {ponderable}]
    n 1: a factor whose effects cannot be accurately assessed;
         "human behavior depends on many imponderables"
    

Imponderable \Im*pon"der*a*ble\, a. [Pref. im- not + ponderable:
   cf. F. impond['e]rable.]
   Not ponderable; without sensible or appreciable weight;
   incapable of being weighed.
   [1913 Webster]
    

Imponderable \Im*pon"der*a*ble\, n. (Physics)
   An imponderable substance or body; specifically, in the
   plural, a name formerly applied to heat, light, electricity,
   and magnetism, regarded as subtile fluids destitute of weight
   but in modern science little used.
   [1913 Webster]
    

69 Moby Thesaurus words for "imponderable":
      airy, asomatous, astral, atomic, bodiless, corpuscular, decarnate,
      decarnated, discarnate, disembodied, embryonic, ethereal,
      evanescent, extramundane, germinal, ghostly, granular, immaterial,
      impalpable, imperceptible, inappreciable, incomprehensible,
      inconceivable, incorporate, incorporeal, indiscernible,
      inestimable, infinitesimal, insensible, insubstantial, intangible,
      invisible, microcosmic, microscopic, molecular, nonmaterial,
      nonphysical, nonsubstantial, occult, otherworldly, phantom,
      psychic, shadowy, spiritual, subatomic, subtle, supernatural,
      tenuous, thin, transmundane, ultramicroscopic, unapparent,
      unappreciable, unconcrete, unearthly, unembodied, unextended,
      unfleshly, unmeasurable, unobservable, unperceivable, unphysical,
      unseeable, unsolid, unsubstanced, unsubstantial, unweighable,
      unworldly, weightless