The Jargon File (version 4.4.7, 29 Dec 2003):

nontrivial
 adj.

    Requiring real thought or significant computing power. Often used as an
    understated way of saying that a problem is quite difficult or impractical,
    or even entirely unsolvable (?Proving P=NP is nontrivial?). The preferred
    emphatic form is decidedly nontrivial. See trivial, uninteresting, 
    interesting.


The Free On-line Dictionary of Computing (30 December 2018):

nontrivial

   Requiring real thought or significant computing power.  Often
   used as an understated way of saying that a problem is quite
   difficult or impractical, or even entirely unsolvable
   ("Proving P=NP is nontrivial").  The preferred emphatic form
   is "decidedly nontrivial".

   See uninteresting, interesting.

   [Jargon File]

   (1995-02-21)