\begin{definition}[Definition:Negation Normal Form]
A propositional formula $P$ is in '''negation normal form''' ('''NNF''') {{iff}}:
* The only logical connectives connecting substatements of $P$ are Not, And and Or, that is, elements of the set $\left\{{\neg, \land, \lor}\right\}$;
* The Not sign $\neg$ appears only in front of atomic statements.
That is $P$ is in '''negation normal form''' iff it consists of literals, conjunctions and disjunctions.
\end{definition}