A slave is a device that fires a flash unit automatically when it detects the light from another flash. It uses a light-sensitive cell, so the flashes do not need to be connected by cables. This makes multi-light setups simpler, in the studio and on location.
Many flashes have a slave mode built in. A simple optical slave can be triggered by any flash, including a camera's pre-flash, so some slaves have a setting to ignore it. Today, radio triggers are often used instead.
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