* Gerund: "Fishing" functions as a gerund, which is a verb form ending in "-ing" that acts as a noun. In this sentence, "fishing" is the object of the preposition "is."
* Predicate Nominative: "Fishing" also acts as a predicate nominative, which is a noun or pronoun that follows a linking verb (like "is") and renames or identifies the subject. In this case, it renames the subject "My greatest pleasure."
In summary: The word "fishing" is a versatile word that can function as both a gerund (a noun form of a verb) and a predicate nominative (re-naming the subject) in the sentence "My greatest pleasure is fishing."