A maturation chamber is a device used by the Borg Collective to accelerate the development of adolescent humanoids until they are ready to service the Collective as a drone. Among those that were placed in such chambers are Annika Hansen (Seven of Nine) and Icheb. (TNG:"Q Who?"; VOY: "Collective")
Children assimilated by the Borg are placed in maturation chambers where they will spend the following seventeen cycles. (VOY: "Mortal Coil")