imponderable

adj

1. Not ponderable; without sensible or appreciable weight; incapable of being weighed.
2. Difficult or impossible to comprehend or evaluate.

noun

1. A substance or body, which is not ponderable, i.e., which not does not having a detectable amount of matter or measurable mass. An imponderable substance or body; specifically, in the plural, a name formerly applied to heat, light, electricity, and magnetism.
2. An imponderable question.
3. A factor that cannot be anticipated.

   “One key imponderable is the attitude of the companies that will no longer have a role in the business. It is worth noting here that several have already thrown the towel in or been gently pushed out, such as Stagecoach and National Express.”