ordinal number

noun3

1. •
   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.
2. •
   A natural number used to denote position in a sequence.

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