(covers information from several alternate timelines)
Temporal reversion is a phenomenon where something would be pushed into an earlier state of entropic decay. In organisms, this would manifest itself as the cellular structure coalescing, quite literally as it aged backwards.
In 2370, the crew of the USS Enterprise NCC-1701-D experienced temporal reversion due to exposure to large amounts of temporal energy being emitted from an anti-time eruption. This had the effect of causing some crewmembers' old scars to heal, the DNA in Lieutenant Commander Geordi La Forge's optic nerves to regenerate, and the death of Nurse Alyssa Ogawa's unborn child, due to the reversion of the embryo's fetal tissue. The timeline in which these events occurred was later negated, however, when Captain Jean-Luc Picard was successfully able to prevent the eruption from occurring in the first place. (TNG: "All Good Things...")
In 2379, Kes was dying at the age of 9. The Doctor hoped to cause controlled temporal reversion through the use of the bio-temporal chamber he developed. Instead, the chamber reactivated dormant chronitons, sending her to progressively earlier points in her life. This temporal reversion and the time travel that accompanied it was stopped in 2373 of a new timeline. (VOY: "Before and After")