a. [ Pref. im- not + ponderable: cf. F. impondérable. ] Not ponderable; without sensible or appreciable weight; incapable of being weighed. [ 1913 Webster ]
n. (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. [ 1913 Webster ]
n. The quality or state of being imponderable. [ 1913 Webster ]