Define:

The mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioural pattern due to the way it can alter the program's running behaviour.

Usage:

Components need to communicate with each other

They communicate via the mediator

The components all have knowledge of the mediator

Advantages:

Hides coordination between components

Component no longer need to know about each other

Mediator turns many to many relationships into one to many/many to one.

Disadvantages:

The mediator can become large and very complicated