The definitions of parts of speech are drawn from various educational sources. A noun is a person, place or thing Many nouns are abstract, and proper names behave like noun phrases, not nouns. Forget about a definition which relies on meaning, which is not part of the syntax. Concentrate instead on substitution. Noun A noun can be inflected to become either plural, possessive, or both.
Some nouns are already plural, such as series and data .
In any case, this is a matter of morpohology alone. The possessive -'s attaches to the right edge of a syntactic structure which is often bigger than a word. The morphology and syntax of the two are completely different. Common nouns are nouns in our grammar. Proper nouns are noun phrases.