Is there a term for nouns that have identical singular and plural forms? For example,

  • sheep
  • fish
  • glasses
  • aircraft/spacecraft



3 Answers 3


According to Wikipedia, some of these are called defective nouns:

Some nouns have no singular form. Such a noun is called a plurale tantum.

For example, glasses, pants, and scissors are all defective nouns because they have no singular form. As these are plurale tantum, the opposite is singularia tantum--nouns with no plural form.

However, examples like sheep and fish are simply irregular plurals.

  • 3
    Slight correction (not intended to be gratuitous): "plurale tantum" is not itself a "plurale tantum". So the sentence As these are plurale tantum... should actually be As these are pluralia tantum... see en.wikipedia.org/wiki/Plurale_tantum for examples of usage as a plural, all in English, not Latin ;#) Oct 13, 2011 at 6:54
  • 4
    I'm unconvinced that 'glasses' should be in the list. A singular eyeglass certainly exists: ask Lord Peter Wimsey. Oct 13, 2011 at 11:23
  • 3
    Fish does have a plural. It is important to distinguish between the collective noun and the plural. For example Matthew 15:36: "And he took the seven loaves and the fishes, ..."
    – Fraser Orr
    Oct 15, 2011 at 16:09
  • 3
    @Fraser: it's not just a collective noun; fish has two plural forms. You can say "two fish", which you couldn't if fish were only a singular or a mass noun. Dec 3, 2011 at 16:59
  • 1
    Yes, they're irregular plurals, but more specifically, they're zero plurals.
    – user28567
    Oct 22, 2013 at 11:34

I think they are called invariable or invariant.

See here for example.

  • 1
    An invariable word is just a word that doesn't change its form in cases where other verbs of the class do: but not necessarily just in view of number. I'm not sure there's actually a commonly used single-word term to refer to invariability just in terms of number. You could talk about a noun being "invariable for number" or "invariable for case" etc. Feb 14, 2011 at 1:15
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    I think you are correct. The NOAD reports invariable as meaning having the same form in both the singular and the plural, e.g., sheep.
    – apaderno
    Feb 14, 2011 at 1:16
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    The term "invariable" may be used in the general sense of not varying, but I think it's specifically used for number as @kiamlaluno says. I have added another reference supporting this.
    – CesarGon
    Feb 14, 2011 at 1:20
  • sure you can use it that way if the context is clear that you're talking specifically about invariability in terms of number. But just bear in mind that if you're working e.g. with a multilingual application, "invariable" has a wider sense. Feb 14, 2011 at 1:55
  • @Neil Coffey: Yes, that's what I said. :-)
    – CesarGon
    Feb 14, 2011 at 9:24

"Glasses" doesn't belong in this list. It has no singular form at all.

You cannot say "a glasses" like you can say "a sheep", "a fish", "an aircraft", and "a spacecraft".

The best you can do is say "a pair of glasses" but now the singular/plural issue gets moved from "glasses" to "pair". Compare if you will "Two pairs of glasses". As mentioned in another answer this is called a plurale tantum.

Now for the rest of the list there is a term that's used in grammar and linguistics to cover this and other cases such as nouns with the same form in both masculine and feminine for languages which have grammatical gender:


"Invariant" doesn't have a special sense for this, it's not linguistic terminology per se, it just happens to be the right word to use for things which don't change.

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