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. (Star Trek: Insurrection)