\begin{definition}[Definition:Rooted Tree/Descendant]
Let $T$ be a rooted tree with root $r_T$.
Let $t$ be a node of $T$.
A '''descendant node''' $s$ of a $t$ is a node such that $t$
is in the path from $s$ to $r_T$.
That is, the '''descendant nodes''' of $t$ are all the nodes of $T$ of which $t$ is an ancestor node.
\end{definition}