In message-oriented middleware solutions, fan-out is a messaging pattern used to model an information exchange that implies the delivery (or spreading) of a message to one or multiple destinations possibly in parallel, and not halting the process that executes the messaging to wait for any response to that message.
Also fan-out in Software construction means the number of classes used by a certain class or the number of methods called by a certain method.
References
Fan-out (software) Wikipedia(Text) CC BY-SA