Imponderable /(?)/

Im·pon·der·a·ble

Imponderable

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

Imponderable

n.
  1. 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. (Physics)