Ordinal number

(grammar) A word that expresses the relative position of an item in a sequence.

"First, second and third are the ordinal numbers corresponding to one, two and three."

A natural number used to denote position in a sequence.

"In the expression a3, the "3" is an ordinal number."

Such a number generalised to correspond to any cardinal number (the size of some set); formally, the order type of some well-ordered set of some cardinality a, which represents an equivalence class of well-ordered sets (exactly those of cardinality a) under the equivalence relation "existence of an order-preserving bijection".