NP-hard refers to a class of problems in computer science that are at least as difficult as the hardest problems in NP (nondeterministic polynomial time). If a problem is NP-hard, it means that there is no known efficient way to solve it, and it is unlikely that one exists. These problems can be very complex and time-consuming to solve, especially as the size of the input increases. An important aspect of NP-hard problems is that they can be used to demonstrate the difficulty of other problems. If a polynomial-time solution is found for any NP-hard problem, it would imply that all problems in NP can also be solved efficiently. This relationship is a central topic in theoretical computer science and has significant implications for fields like cryptography and optimization.