User blog comment:Alejandro Magno/My Ordinal is Smaller!/@comment-1605058-20140922144957/@comment-1605058-20140922150316

Okay.

Ordinal numbers are order types of well-ordered sets. Order type of empty set ordered by set inclusion is 0. This is obviously a well-order, because we have no elements which can form an infinite descending sequence, and it's of course a linear total order.

Thus 0 is order type of a well order, so, by definition, it's an ordinal.