Not ponderable; without sensible or appreciable weight; incapable of being weighed.

a factor whose effects cannot be accurately assessed; "human behavior depends on many imponderables"

difficult or impossible to evaluate with precision; "such imponderable human factors as aesthetic sensibility"

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.