An abstract noun is a type of noun that refers to ideas, qualities, or conditions that cannot be perceived through the five senses. Unlike concrete nouns, which denote physical objects like apple or car, abstract nouns represent concepts such as happiness, freedom, or justice. These nouns express emotions, states of being, or characteristics. Abstract nouns can be formed from adjectives or verbs. For example, the adjective happy can become the abstract noun happiness, while the verb to create can lead to the noun creation. Understanding abstract nouns helps in expressing complex thoughts and feelings in language.