Category:English invariant nouns
Note: These are not to be confused with pluralia tantum (such as bagpipes) or with uncountable nouns (also known as mass nouns and non-count nouns; invariant nouns can still be counted as in "one sheep, two sheep").
A related class are certain possessives that are (headless) elliptical noun phrases. (The chemist's is closed on Sundays. The chemist's were closed on Sundays.) See Appendix:Special uses of possessives in English.
Invariant use of non-invariant nouns 
Sometimes in English, the singular form of a non-invariant noun may be used to denote a plural. This should be carefully distinguished from true invariants, where there is no plural form, as in the case of 'sheep'.
Generally, this happens with only a very few kinds of noun :
Pages in category "English invariant nouns"
The following 70 pages are in this category, out of 70 total.