\begin{definition}[Definition:Literal]
A '''literal''' is either:
:an atom $p$ of propositional logic, that is, a statement, or
:the negation $\neg p$ of an atom $p$.
In the language of propositional logic, these correspond to:
:a letter $p$;
:the WFF $\neg p$, where $p$ is a letter.
\end{definition}