Definition:Ordinal/Definition 2

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Let $A$ be a set.

Then $A$ is an ordinal if and only if $A$ is:

epsilon-connected, that is:
$\forall x, y \in A: x \ne y \implies x \in y \lor y \in x$


The class of all ordinals can be found denoted $\On$.

In order to indicate that a set $S$ is an ordinal, this notation is often seen:

$\Ord S$

whose meaning is:

$S$ is an ordinal.

Thus $\operatorname {Ord}$ can be used as a propositional function whose domain is the class of all sets.

According to 1993: Keith Devlin: The Joy of Sets: Fundamentals of Contemporary Set Theory (2nd ed.), it is common practice in set theory to use lowercase Greek letters $\alpha, \ \beta, \ \gamma, \ldots$ for ordinals.