\begin{equation} NP = \bigcup_{k \in N} \text{NTIME}\qty(n^{k}) \end{equation}
Meaning, these are problems with the property that once you “have” the solution, its “easy” to verify the solution.
verifier formulation of NP
\(L \in \text{NP}\), if there exists a Polynomial Time turing machine named \(V\) (called a “verifier”) such that:
\begin{equation} X \in L \Leftrightarrow \exists\ w \in \qty {0,1}^{\text{poly}\qty(|x|)} V(x,w) = 1 \end{equation}
that is, “yes instances have efficiently checkable certificates/“proofs”