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

NP-hard

    A set or property of computational search
   problems.  A problem is NP-hard if solving it in polynomial
   time would make it possible to solve all problems in class
   NP in polynomial time.

   Some NP-hard problems are also in NP (these are called
   "NP-complete"), some are not.  If you could reduce an NP
   problem to an NP-hard problem and then solve it in polynomial
   time, you could solve all NP problems.

   See also computational complexity.

   [Examples?]

   (1995-04-10)