Final answer:
An entity that joins two other entities in a database is called an associative entity, used for many-to-many relationships.
Step-by-step explanation:
An entity that joins two other entities in a database is called an associative entity. It is used when there is a many-to-many relationship between the two entities, meaning that each entity can be related to multiple instances of the other entity.
For example, let's consider a database for a bookstore. We have two entities: 'Books' and 'Authors'. Since an author can write multiple books and a book can have multiple authors, we need an associative entity to represent this relationship. This entity would contain the necessary attributes and mappings to connect the two.