not perceptible, not detectable, too small in magnitude to be observed
beyond the purview of man; too great and all-encompassing to be perceived.
Not ponderable; without sensible or appreciable weight; incapable of being weighed.
Difficult or impossible to comprehend or evaluate.
An imponderable question.
A factor that cannot be anticipated.
An imponderable substance or body; specifically, in the plural, a name formerly applied to heat, light, electricity, and magnetism.