In linguistics, and in particular the context of grammar, a gerund is a present participle verb which acts as a noun by adding -ing to the end. Some grammarians, however, refer to these as verbal nouns ending in -ing, for example Sidney Greenbaum in An Introduction to English Grammar (1991).
For example:
- I like writing (direct object)
- Programming is fun. (subject)
- Jane was swimming in the sea. (The participle verb here functions as an adjective and is not a true gerund.)