Full Idea
The notion of an ordinal number is a set-theoretic, and hence non-arithmetical, concept.
Gist of Idea
The concept of 'ordinal number' is set-theoretic, not arithmetical
Source
Leon Horsten (Philosophy of Mathematics [2007], §2.3)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.7