The process of restoring decayed dilithium crystals to working order, thought impossible in the 23rd century until Spock and Scott achieved it with high-energy photons from a nuclear reactor in Star Trek IV: The Voyage Home.
Dilithium crystals gradually break down in use, and in Kirk's era a ship with decrystallised dilithium simply needed new crystals. Recrystallisation, the rebuilding of the crystal structure, was regarded as beyond the science of the time.
In Star Trek IV: The Voyage Home, the captured Klingon Bird-of-Prey that carries Kirk's crew back to 1986 arrives with its crystals badly drained by the journey. Spock proposes a method drawn from older theory: bombarding the crystals with high-energy photons of the kind given off by a 20th-century fission reactor. Chekov and Uhura slip aboard the nuclear-powered aircraft carrier Enterprise to gather them, which leads to Chekov's capture and injury.
The collected energy restores the crystals well enough for the return trip. By the 24th century recrystallisation takes place routinely inside the warp core.