A regeneration alcove, also known informally as a Borg alcove or simply alcove or slot, was a device used by drones for regeneration. When the Borg drones were not needed for immediate tasks, it was used as a drone storage device by the Borg Collective.
The alcoves themselves, were initially described as "slots along the wall, kind of like compartments. There are two Borg in each." Data then theorized that "the Borg are somehow interconnected through these slots and are working collectively." (TNG: "Q Who")
A regeneration cycle could be interrupted without the drone ever knowing, using various technology that prevented the drone from "waking" when the cycle was interrupted. The Hansens used this technique to examine Borg drones by beaming a drone from its alcove to their ship. (VOY: "Dark Frontier")
An alcove was sized to allow the drone to fit inside. It was sized slightly wider and higher than a drone, with a small raised platform for the drone to stand upon. A green disc large enough to accommodate the drone's head was positioned at the height of the drone, pulsating visually with various patterns, some static and others dynamic, usually green and white in color. (VOY: "Dark Frontier")
Once positioned into the alcove, a drone's mechanical parts established a connection with the alcove's systems (much the same way a doctor would monitor a patient with a biobed. The alcove then established a connection in one of two ways: either through a port resembling an arm rest onto which a drone would connect its arm, or by means of tubes which connected directly from the wall into various ports in the drone exo-plating. (TNG: "Q Who"; VOY: "Scorpion")
When a drone's cortical node malfunctioned a regeneration cycle could not be started. If a drone remained in an unregenerate state for too long, it lost motor control, and eventually became unconscious; a state very undesirable to the Borg. If a Borg drone was unable to continue to fully regenerate as it needed (due to injury or malfunction), it would be removed from its alcove by other drones and its components were salvaged for reuse. (VOY: "Dark Frontier")
With knowledge of Borg technology, alcoves could be integrated directly into Starfleet vessels to serve the same purpose of their counterparts on Borg vessels. When this was done they would require over thirty megawatts of power to run each alcove. (VOY: "Dark Frontier") Or if needed, a Borg drone could regenerate by means of a converted Federation power conduit which would function similar to an alcove. (TNG: "I Borg")
The interface for a regeneration alcove offered many advantages, as noted by Commander William T. Riker, "with speed being the most obvious," and with such an interface "the ship literally just thinks what it wants, and then it happens."
When the USS Enterprise-D first encountered a Borg cube in 2365 that was later boarded by Riker and Data, there was no indication of life aboard the cube. Data surmised that "Perhaps because this ship was scanned for individual life signs. Apparently when they are in these slots, they become part of the whole and no longer read as separate life forms." (TNG: "Q Who")
A Borg drone would enter its regenerative state similar to stasis for a period of time called a regeneration cycle. A regeneration cycle would vary in length based on factors such as: how long it had been since the drone last entered an alcove; what species and age the drone was; was the drone injured and was the drone needed for assignment or task. A typical regeneration cycle took a full 6 hours to complete. (VOY: "Child's Play") When part of the collective a drone could go without regeneration for roughly two hundred hours before it suffered serious ill-effects, however a former Borg drone severed from the collective such as Seven of Nine must regenerate at least three hours a day. (VOY: "Hunters")
Although during regeneration a drone would enter a mental state similar to that of sleep, it remained far from unconsciousness. Drones who were disconnected from the Borg Collective and hive mind were able to stop the regeneration cycle themselves at their desire, although an incomplete regeneration cycle is likened to being woken from slumber and can have an effect of grogginess. (VOY: "Child's Play") It may also appear to the casual observer that the drone appeared to be "in some kind of stasis." (TNG: "Q Who")
A drone that was functioning normally and part of the Hive Mind did not dream; instead, a drone's mental capacity was apportioned to various tasks. These tasks sometimes involved the control of nanites. For instance, a drone might be utilized to repair damage to a vessel or to heal injured comrades while remaining semi-conscious in the alcove. In some situations, a drone would perform various tasks that were directly connected to other drones or the ship's systems, while remaining connected to the alcove. (TNG: "Q Who"; VOY: "Unity")
In 2377, the Borg discovered that drones with a genetic mutation used their alcoves to create Unimatrix Zero, in which they could interact with each other while regenerating, although the memories and experiences gained within it would not stay with the drone when the regeneration cycle ended. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")