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]

WordNet (r) 3.0 (2006):

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"