The definition of NP-hard problem in Wikipedia is : NP-hardness (non-deterministic polynomial-time hard), in computational complexity theory, is a class of problems that are, informally, "at least as hard as the hardest problems in NP".
There are NP-hard problems not in NP class. My question (more curiosity) concerns the NP in the definition. Since a NP-hard maybe is not in NP class, why we call it NP-hard?
When we say we have a NP-complete problem, then this problem is indeed in NP class.