A noun is a word that functions as the name of some specific thing or set of things, such as living creatures, objects, places, actions, qualities, states of existence, or ideas. However, noun is not a semantic category, so that it cannot be characterized in terms of its meaning. Thus, actions and states of existence can also be expressed by verbs, qualities by adjectives, and places by adverbs.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/noun-and-pronoun