What is the name of the connection when a film has many actors and one actor appears in many films?

The relationship where a film has many actors and an actor appears in many films is called "many-to-many" in relational model terminology.

In the context of data analysis or databases, actors and films are related by a many-to-many relationship: one film can have many actors, and one actor can appear in many films.

More generally, it is simply multiple connections between two sets of objects.