Regeneration
Regeneration was a process of repair or renewal. Regenerating damaged tissue was an important field in medicine. In addition, regeneration was a natural activity undertaken by some species during their resting cycle, analogous to sleep.

Medical usage 

In 2377, The Doctor found a way to adapt Borg regeneration techniques to treat Dr. Lewis Zimmerman's acute subcellular degradation. ()

Natural usage 

The nucleogenic cloud being encountered by the  in 2371 in the Delta Quadrant had the ability to regenerate. To assist this regeneration process, Voyager transfered a nucleonic beam into the lifeform. ()

Borg 
Borg drones regenerated in Borg alcoves. These alcoves were found aboard Borg vessels and also aboard the Voyager, where Seven of Nine and later the Borg children regenerated. (; )

A Borg drone can function for up to 200 hours before needing to regenerate. Seven of Nine, once she was separated from the Collective, needed to regenerate for at least three hours a day to maintain her health. ()

Borg drones also regenerate outside of their alcoves when nanoprobes repair the drone itself, with their cybernetic components being undergoing maintenance and repair on a microscopic level, as they could also repair cells. ()

When Seven of Nine discussed Icheb's regenerative needs to Leucon, she explained how she had adapted a neural transceiver to interface with a portable regenerator. Seven figured the device had enough  to complete one full  for Icheb; before adding that "[i]t's important that Icheb regenerate for six uninterrupted hours." It can be presumed she provided Azan, Rebi and Mezoti with such devices when they left Voyager. ()

Borg drones could also regenerate and repair damage to a Borg ship, such as a Borg cube or a Borg sphere from the alcoves. (; ) 

Sometimes, disturbing thoughts could interrupt a Borg's regeneration cycle. ()

Changelings 

A Changeling could only remain in solid form for sixteen to eighteen uninterrupted hours, at which time they needed to revert to a gelatinous state. (; )

Failure to do this could lead to damage to their morphogenic matrix, which manifested as discoloration and "flaking off" of mass. ()

Jadzia Dax often entered Odo's quarters while he was regenerating and moved several pieces of his furniture, only by a few centimeters, knowing this would irk the constable. ()

After Odo (serving as umpire) called a strike in a baseball game between the Niners and the Logicians, Benjamin Sisko claimed that Odo must have been regenerating and not watching the game at all. ()

Gideons 
Gideons were continuously regenerating their bodies, which meant that, save for accident and disease, they were immortal. ()

Species 8472 
In 2373, Lieutenant Commander Tuvok detected that damage to a wall inside a Species 8472 bio-ship was caused by a Borg disruptor beam. However, the wall appeared to be regenerating itself. ()

See also 
 Bone regenerator
 Cellular regeneration and entertainment chamber
 Dermal regenerator
 Eco-regeneration
 Myelin regenerator
 Neuro-regeneration procedure
 Regenerative force field
 Regenerative shield
 Regenestasis
 Rejuvenation
 Vascular regenerator

de:Erholung
Physiology
Sleep