NOUN (1)

1. a factor whose effects cannot be accurately assessed;
- Example: "human behavior depends on many imponderables"


ADJECTIVE (1)

1. difficult or impossible to evaluate with precision;
- Example: "such imponderable human factors as aesthetic sensibility"

The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

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]

Moby Thesaurus II by Grady Ward, 1.0:

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