A common noun is a type of noun that refers to general items rather than specific ones. It is the generic name for a person, place, or thing in a class or group. Common nouns are not capitalized unless they appear at the start of a sentence or in a title. They can be concrete, abstract, or collective.

Here are some key characteristics of common nouns:

• They describe a type of person, thing, or place, or a concept.
• They are not capitalized unless they appear at the start of a sentence or in a title.
• They are general and usually name classes of things, people, and places rather than specific ones.
• They can be modified by articles, determiners, and adjectives.
• They can include the names of different jobs, plants and animals, geographical features, ideas, objects, and many other things.

Examples of common nouns include "city," "mountain," "cat," "book," and "teacher". Common nouns are defined by contrast with proper nouns, which are the names of specific individuals, things, places, companies, etc. Proper nouns are always capitalized and typically not modified by articles, determiners, or adjectives.