But what about earthquakes and concerts and wars, values and weights and costs, famines and droughts, redness and fairness, days and millennia, functions and purposes, craftsmanship, perfection, enjoyment, and finesse?
—Ray Jackendoff, Foundations of Language: brain, meaning, grammar, evolution
I learned in school that a noun is a word that names a person, place or thing.
A few years after that, the definition changed. In hindsight this seems creepy. It happened twice. I don't remember any explicit discussion or even acknowledgment of the change. We would do nouns one way one year, and when that time came around the next year, we would have different textbooks with a different definition. I remember the changes: first “event” and later “idea” were added to the list ...bringing nouns like earthquakes and purposes in from the cold, I guess. We regret the omission, etc.
I didn't know this until a couple days ago, but linguists apparently consider this whole approach to parts of speech hopelessly, fundamentally broken. Morally bankrupt, in fact. That a child is taught the ”person, place or thing“ definition approximately once every 12 seconds preys on the linguist's soul. It causes him to make awkward scenes at parties. Even the funny papers are bristling with painful reminders of this horrible truth.
I never noticed before, but there is a problem or two with this whole “person, place or thing” thing. All the most common words for people (you, I, he, she, they) and things (this, that, these, those, it) are pronouns, while all the most common words for places (here, there, in, out, up, down, to, from, and on and on) are adverbs and prepositions. All the other definitions I learned for parts of speech are bogus, too. I learned that “action words” are verbs; but homocide, defenestration, and touchdown are all nouns. (So is pirouette. My wife didn't believe me.) I learned that prepositions tell about relationships, particularly spacial relationships; but proximity and distance are nouns (and cover and surround are verbs!). I learned that words that describe properties of things are adjectives; but weight, beauty, shape, and color are nouns.
So what is the definition of a noun, exactly? Well, I'll tell you. I don't know. Strangely, I don't think linguists like to say! Here's a pretty good near miss by Geoffrey K. Pullum, writing in Language Log:
The way to tell whether a word is a noun in English is to ask questions like: Does it have a plural form (the terrors of childhood)? Does it have a genitive form (terror's effects)? Does it occur with the articles the and a (the terror)? Can you use it as the main or only word in the subject of a clause (Terror rooted me to the spot), or the object of a preposition (war on terror)? And so on. These are grammatical questions. Syntactic and morphological questions. Not semantic ones.
A bit vague, isn't it? That's way above average, though. Here's an honest attempt; it starts with “A noun is a member of a syntactic class…”. Until I edited it, Wikipedia's article on nouns started, “In linguistics, a noun or noun substantive is a lexical category which is defined in terms of how its members combine with other kinds of expressions.”
There's an interesting twist to how all this gets bootstrapped in the toddler brain. All the first words you learn are nouns, words for people and things in your little one-year-old world. You'll be able to put words together into sentences before you master any pronouns. That is, at the time when you're learning the basic grammar of the language, there is a semantic distinction between the nouns you know and all other words. The values and weights and costs come later.