https://encyclopediaofmath.org/wiki/Ordinal_number Ordinal number The order type of a well-ordered set. This notion was introduced by G. Cantor in 1883 (see [2]). For instance, the ordinal number of the set N of all positive integers, ordered by the relation ≦, is ω. The ordinal number of the set consisting of 1 and numbers of the form 1-1/n where n∈N, ordered by the relation ≦, is ω+1.