\begin{definition}[Definition:Atom of Sigma-Algebra]
Let $\struct {X, \Sigma}$ be a measurable space.
Let $E \in \Sigma$ be non-empty.
$E$ is said to be an '''atom (of $\Sigma$)''' {{iff}} it satisfies:
:$\forall F \in \Sigma: F \subsetneq E \implies F = \O$
Thus, '''atoms''' are the minimal non-empty sets in $\Sigma$ with respect to the subset ordering.
\end{definition}