Definition 2022


ordinal number

See also: ordinal-number



ordinal number (plural ordinal numbers)

  1. (grammar) A word that expresses the relative position of an item in an ordered sequence.
    First, second and third are the ordinal numbers corresponding to one, two and three.
  2. (arithmetic) A number used to denote position in a sequence.
    In the expression a3, the "3" is an ordinal number.
  3. (mathematics) A generalized kind of number to denote the length of a well-order on a set.


  • (grammar) ordinal, ordinal numeral



Usage notes

On ordinal number usage:

  • Ordinal numbers are generally construed as adjectives preceding (countable) nouns in singular:
    eleventh day
  • Ordinal numbers are used for fractions as (countable) nouns, and for exponents usually followed by power and that numeral:
    two fifths,
    two to the minus twenty-first power,
    six to the third,
  • Ordinal numbers are generally considered to be ordered from high to low, so that first place is considered highest, and fifth is lower than second. Degree is an exception.
  • Ordinal numbers corresponding to numbers higher than 20 use cardinal numbers for all the places preceding the final ordinal part:
    twenty-first or 21st, occasionally XXI
    one hundred fifteenth or 115th , occasionally CXV
    thirty-three thousandth or 33,000th
  • If an ordinal is followed by a plural noun, the two word phrase refers to a set of items described by the phrase in singular. For example second homes refers to a set of homes which are considered a "second home."
    Many households have third cars.

See also