What is the name for a word that is both singular and plural? [duplicate]
Solution 1:
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.
Solution 2:
I think they are called invariable or invariant.
See here for example.