Lexeme is the term coined by Crystal to cover this, though it is not specific to inflections of nouns.
A lexeme is a unit of lexical meaning that underlies a set of words
that are related through inflection. It is a basic abstract unit of
meaning, a unit of morphological analysis in linguistics that
roughly corresponds to a set of forms taken by a single root word. For
example, in English, run, runs, ran and running are forms of
the same lexeme, which can be represented as run.
One form, the lemma (or [base form or] citation form), is chosen by convention as the
canonical form of a lexeme. The lemma is the form used in dictionaries
as an entry's headword. Other forms of a lexeme are often listed later
in the entry if they are uncommon or irregularly-inflected.
Lexeme An alternative term is lexical item. Lexemes (or lexical items) are the basic units of meaning in a language. Dictionaries list
them. There’s no harm, of course, in following the long-established
tradition, and continuing to say ‘look the word up in a dictionary’,
as long as students realise that what they’re really doing is looking
up a lexeme. Most of the time, you can carry on talking about ‘words’,
and there’ll be no ambiguity. But lexeme is a handy term to have
available when students need to talk about such matters as variant
forms (the go problem) and idioms.
An idiom is a lexeme, because regardless of the number of words it
contains it expresses a single meaning, as in the case of kick the
bucket (which incidentally derives from old pig-slaughtering
practices, when pigs were hung from beams).
Other examples of variant forms: plurals (boy and boys are the same
lexeme), comparison (big, bigger, and biggest are the same lexeme),
possession (girl and girl’s are the same lexeme).