A common noun is a word that names any general person, place, thing, or idea rather than a specific one. Words like teacher, city, book, and happiness are all common nouns because they could refer to any teacher, any city, any book, or the general concept of happiness.

Common nouns are the opposite of proper nouns, which name specific people, places, or things and always start with capital letters. For example, teacher is a common noun, but Mrs. Johnson is a proper noun. The word city is common, but Chicago is proper. You can have a dog (common noun) named Rover (proper noun).

Common nouns usually don't need capital letters unless they start a sentence. If you write “The mountain was tall,” mountain stays lowercase because it's a common noun. But if you write “Mount Everest was tall,” Mount Everest gets capital letters because it names a specific mountain.

Understanding common nouns helps you see how language categorizes the world. We use common nouns constantly to talk about types and categories of things, while proper nouns help us identify the specific members of those categories.