\begin{definition}[Definition:Extension of Branch of Propositional Tableau]
Let $T$ be a propositional tableau, and let $\Gamma$ be a branch of $T$.
Let $S$ be an extension of $T$, and let $\Gamma'$ be a branch of $S$.
Then $\Gamma'$ is an '''extension of $\Gamma$''' {{iff}} $\Gamma \subseteq \Gamma'$.
Informally, a branch may be '''extended''' by successively adding children (with WFFs attached to them) to its leaf node, one at a time.
\end{definition}