# ordinal number

## English

### Noun

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.

#### Synonyms

• (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.
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