Definition:Rooted Tree/Descendant

Definition
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.