A noun is a part of speech that is used to name or identify a person, place, thing, or idea. Nouns can play different roles in a sentence, such as the subject, direct object, indirect object, subject complement, object complement, appositive, or modifier. Some examples of nouns include "cat," "bicycle," "Taylor Swift," and "Kuala Lumpur".

Nouns can be categorized into different types, such as common nouns, proper nouns, abstract nouns, collective nouns, and concrete nouns. Common nouns are the generic name of an item in a class or group and are not capitalized unless appearing at the beginning of a sentence or in a title. Proper nouns, on the other hand, refer to specific people, places, or things and are capitalized. Abstract nouns refer to ideas, concepts, or emotions, while concrete nouns refer to physical objects that can be touched, seen, smelled, tasted, or heard. Collective nouns refer to groups of people, animals, or things, such as "team," "herd," or "flock".

Nouns can also be used to modify other nouns, functioning like adjectives, and are often called attributive nouns. For example, "speed" is normally a noun, but it can function as an adjective to modify "demon" in the phrase "speed demon".

In summary, a noun is a word that names or identifies a person, place, thing, or idea, and it can play different roles in a sentence. Nouns can be categorized into different types based on their characteristics, such as common nouns, proper nouns, abstract nouns, collective nouns, and concrete nouns.