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

complementary nondeterministic polynomial
Co-NP

    (Co-NP) The set (or property) of problems with a
   yes/no answer where the complementary no/yes problem takes
   nondeterministic polynomial time (NP).

   For example, "Is n prime" is Co-NP and "Is n not prime" is NP,
   since it is only necessary to find one factor to prove that
   n is not prime whereas to prove that it is prime all possible
   factors must be eliminated.

   (2009-05-21)