In computer networking, master/slave is a model for a communication protocol in which one device or process (known as the master) has total control one or more other devices or processes (known as slaves). Once the master/slave relationship is established, the direction of control is always from the master to the slave(s).
The term master/slave has its roots in engineering and has been in use since the industrial revolution. In 2003, a worker in California filed a discrimination complaint with the county's Office of Affirmative Action Compliance after seeing the labels "master" and "slave" on a videotape machine. In response, the County of Los Angeles asked manufacturers, suppliers and contractors to remove labels that could be interpreted as discriminatory or offensive. Since that time, the terms "master/minion" and "primary/secondary" are increasingly being used in place of master/slave in technical documentation.
Other important communication protocol models in digital communication include the client/server model, in which a server program responds to requests from a client program, and the peer-to-peer model, in which either of the two devices involved can initiate a communication session.