The fusion era was a period of time on Earth in which nuclear fission reactors were replaced by fusion-based reactors. This era began sometime after 1986.

That year, while formulating a possible solution to re-crystallize the HMS Bounty's dilithium crystals, Spock recalled the events leading to this era and surmised that a fission reactor could still be found at this juncture. (Star Trek IV: The Voyage Home)