Regeneration

Regeneration is 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.

Regeneration in medicine

 * to be added

Borg
Borg drones regenerated in Borg alcoves. Borg alcoves were used on the USS Voyager to allow Seven of Nine to regenerate. Later, they were also used by the Borg children and adolescents that joined the crew. 

Borg drones could also regenerate and repair damage to a Borg ship, such as a Borg cube or a Borg sphere from the alcoves. (The effect of the Borg cube's regeneration in Q Who was created by building a scale replica of a Borg cube's 'wall', melting it with a blowtorch, and then playing the footage in reverse during filming).

Changelings
A Changeling could only remain in solid form for sixteen 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.