Which of the following defines a noun?

The definition of a noun as "a word used to name a class of things or a specific entity" is accurate because nouns serve the fundamental purpose of identifying people, places, things, or ideas. This encompasses both generic terms (like "fruit" or "city") that represent a category and proper nouns (like "London" or "Sarah") that denote specific instances. Understanding this definition is crucial for recognizing how nouns function in sentences, as they serve as subjects or objects and help convey meaning.

The other options describe different parts of speech or concepts. For instance, a word describing an action refers to verbs, while a word that substitutes for a noun relates to pronouns. Meanwhile, a vivid comparison is characteristic of similes or metaphors, which are literary devices, not nouns. Therefore, clearly grasping what a noun encompasses is essential in studying grammar and understanding sentence structure.