[kazinator]

An empty set is countable; it has an empty mapping to the natural numbers. Its cardinality is zero.

[lupire]

And its ordinalitiy is also 0.

Standard construction of ordinals is that each ordinal is the set of all its predecessors. (0 has no predecessors , hence 0 is the empty set.) (And so finite ordinals have the same ordinaliity as cardinality).

[pwdisswordfish9]

Show me a mathematical text where ‘ordinality’ is defined.