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.
The nucleogenic cloud being encountered by the USS Voyager in 2371 in the Delta Quadrant had the ability to regenerate. To assist this regeneration process, Voyager transfered a nucleonic beam into the lifeform. (VOY: "The Cloud")
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. (VOY: "The Haunting of Deck Twelve"; Star Trek: First Contact)
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. (VOY: "Hunters")
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. (ENT: "Regeneration")
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 power to complete one full cycle for Icheb; before adding that "[i]t's important that Icheb regenerate for six uninterrupted hours." (VOY: "Child's Play")
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. (DS9: "Take Me Out to the Holosuite")
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. (VOY: "Scorpion")