Nuclear fission is a nuclear reaction in which a heavy nucleus splits into two lighter nuclei, releasing a considerable amount of energy.
In the 20th and 21st century, nuclear fission was used as a power source in nuclear fission reactors, but also as a devastating weapon. Nuclear fission produces beta radiation. (Star Trek IV: The Voyage Home; Star Trek: First Contact; DS9: "Little Green Men")
The NASA explorer Charybdis, which operated in the 2030s, had a block D gaseous-core fission reactor for a propulsion system. This allowed the spaceship to produce a net velocity of 7.44×103 NVP. (TNG: "The Royale", Star Trek: The Next Generation - The Continuing Mission) The thought of nuclear fission taking place in a planet's atmosphere shocked Quark and Rom when they learned about it from Nog in 1947. (DS9: "Little Green Men")