Metaphasic radiation

From Memory Alpha, the free Star Trek reference.

Metaphasic radiation is a form of energy which, under certain conditions, reverses the aging process of most humanoid species, thereby prolonging the life of those who are exposed to it. Metaphasic radiation also has remarkable regenerative properties, as in the case of Geordi La Forge's eyes, and also has rejuvenating properties, increasing the metabolism and energy levels of the crew of the USS Enterprise-E temporarily. However, this requires constant exposure.

High concentrations of metaphasic radiation were discovered to be contained within the rings of the planet Ba'ku in the Briar Patch in 2375. An offshoot race of the Ba'ku, the Son'a, who were desperate to harvest the radiation so as to expand their lifespans. However, since the Ba'ku planet is in Federation territory, they enlisted the help of Matthew Dougherty to have the Council agree to have the radiation harvested, under the pretense of medical purposes. Due to the fact that the loss of the radiation would mean making Ba'ku uninhabitable the Enterprise was able to foil the plan. (Star Trek: Insurrection)