Not able to be perceived by the senses (especially by touch); intangible or insubstantial; not easily grasped or understood.
Difficult or impossible to comprehend or evaluate.
Not ponderable; without sensible or appreciable weight; incapable of being weighed.
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.