slave
n. countablen. a person who is legally owned by someone else and forced to work for no money. In technology, it describes a machine or part that is controlled by another main part.
n. a person held in servitude as the property of another; also, a device or process that is controlled by another (the master) in a hierarchical system.
The computer uses a slave drive to store extra data.
In professional photography, a slave flash triggers automatically when it detects the light from the main camera flash.
Historians continue to study the systemic impact of the transatlantic trade on those who were held as slaves and the societies that profited from their forced labor.
In technical contexts, it is almost always paired with 'master' to describe a control relationship.