The anal fin is a distinctive feature of most fishes. It is a fin located on the underbelly, just behind the anus. Although the term 'anal fin' is commonly used to describe this feature, there are other synonyms that can be used. Some of the most commonly used synonyms include ventral fin, ventral finlet, ventral keel, ventral fin ray, and pelvic fin. These terms are all used to describe the same fin that is situated on the underside of the fish and it is important to know these synonyms as they are often used interchangeably in scientific and angling communities.