A gerund is a verb form that ends in -ing and functions as a noun in a sentence. This means that while it originates from a verb, when it takes the -ing form, it can act as the subject, object, or complement within the sentence. For example, in the sentence "Swimming is fun," the word "swimming" functions as the subject and is a gerund.
In contrast, other options do not accurately define what a gerund is. A musical genre refers to a category of music, which is unrelated to grammatical structures. A literary device used in poetry could refer to various techniques like metaphor or simile, which do not pertain to verb forms. An adjective describing an ongoing action doesn’t capture the essence of a gerund, as gerunds specifically serve the role of nouns rather than adjectives. Thus, the first option accurately encapsulates the definition of a gerund.