Source : Webster's Revised Unabridged Dictionary (1913)

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.

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.

Source : WordNet®

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