A cell is A small room designed to lock up and hold a captive. For BDSM purposes, cells are smaller than the average prison cell, since the captive will only stay there for the duration of a scene, which usually doesn’t go longer than a few hours. As they are meant for short-term use, cells occasionally don’t even have furniture or are only sparsely furnished. The cell creates a sense of isolation and also has the effect of sensory deprivation since the captive can’t see or hear what’s going on outside the cell door.
For safety reasons, it’s common to equip cells with baby monitors or webcams to supervise that the captive is not hurt while in the cell, and also supervise them without their knowledge.