Definition of imponderables:

part of speech: noun plural

Those agents or forces which are destitute of weight, as light, heat, electricity, and magnetism.

Usage examples:

• The waiting was the most difficult; too many imponderables did not allow for unemotional thinking. Andre Alice Norton in "The Defiant Agents".

• It took not so long as it takes to describe it, but it was an analysis of imponderables equal to any of Bunsen's with the spectroscope. Oliver Wendell Holmes, Sr. (The Physician and Poet not the Jurist) in "The Complete PG Works of Oliver Wendell Holmes, Sr.".