Imponderable
Imponderable , adjective
[Prefix im- not + ponderable: compare French impondérable.]
Not ponderable; without sensible or appreciable weight; incapable of being weighed.
Imponderable , noun
(Physics) 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.