ordinal number

名词 Noun

(grammar) A word that expresses the relative position of an item in a sequence.
(arithmetic) A natural number used to denote position in a sequence.
(set theory) 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".