\begin{definition}[Definition:Separated by Function]
Let $\left({X, \vartheta}\right)$ be a topological space.
Let $A, B \subseteq X$.
Then $A$ and $B$ are '''separated by function''' iff there exists an Urysohn function for $A$ and $B$.
$A$ and $B$ may well be singleton sets $A = \left\{{a}\right\}, B = \left\{{b}\right\}$.
In this case $a$ and $b$ are '''separated by function''' iff $A$ and $B$ are '''separated by function'''.
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\end{definition}