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