imponderable

Short definition from Wiktionary

Adjective

1. 1
   Not ponderable; without sensible or appreciable weight; incapable of being weighed.
2. 2
   Difficult or impossible to comprehend or evaluate.

Noun

1. 1
   A substance or body, which is not ponderable, i.e., which not does not having a detectable amount of matter or measurable mass. An imponderable substance or body; specifically, in the plural, a name formerly applied to heat, light, electricity, and magnetism.
2. 2
   An imponderable question.
3. 3
   A factor that cannot be anticipated.