I remember myself on my first year of CS, set theory classes, at the whiteboard, trying to make a proof, but there something I was not able to prove at all, so I said 'it's trivial' and the doctor said 'yeah, it's trivial' and we went further.
In a literal sense, it very well may have been trivial, even if neither you nor the professor would have been able to easily show it.
The one I've always flown with is, trivial means (1) a special case of a more general theory (2) which flattens many of the extra frills and considerations of the general theory and (3) is intuitively clear ("easy") to appreciate and compute.
From this perspective, everything is trivial from the relative perspective of a god. I know of no absolute definition of trivial.