\textbf{Definition - Pure Literal.} A literal is \emph{pure} if its negation does not appear in the formula.

Consider the following clauses $c_1$ and $c_2$:
\begin{itemize}
  \item $c_1 = \{a, b, \neg c\}$
  \item $c_2 = \{\neg b, \neg c, d\}$
\end{itemize}

$\neg c$ is a pure literal, since it does not appear in the positive phase.