What is the grammatical term for a verb ending in "ing" that serves as a noun?

The term for a verb ending in "ing" that functions as a noun is known as a gerund. Gerunds are derived from verbs and maintain the action or meaning of the verb while performing the role of a noun in a sentence. This includes uses like "Swimming is fun," where "swimming" acts as the subject of the sentence.

In contrast, infinitives refer to the base form of a verb, often preceded by "to," such as "to run" or "to think," and while participles also end in "ing," they are used as adjectives to modify nouns, like in "running water." Adverbs modify verbs, adjectives, or other adverbs but do not serve as nouns. Understanding these distinctions clarifies how each form functions in a sentence structure.