class NP-hard

Definition

Define language $L'$ as NP-hard if $L \leq_p L'$ for every $L \in$ NP. (polynomial-time reduction from $L$ to $L'$)

(see NP, also note that $\leq_p$ means polynomial-time reduction)

References

1. S. Arora, B. Barak. Computational Complexity: A Modern Approach, Cambridge University Press, 2009, p. 42.