Is there a term for nouns that have identical singular and plural forms? For example,
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.
I think they are called invariable or invariant.
See here for example.
"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.