Nuclear fission

Nuclear fission is the splitting of a heavy nucleus is split into two lighter nuclei, producing energy in the process. Fission was the first nuclear reaction that humans were able to create artificially, in the first half of the 20th century.

Even though fusion had a number of advantages to fission, humans were generally unable to create fusion artificially for a number of years, leading humans to use fission reactors quite extensively during the 20th and 21st centuries. One of the main uses of fission reactors was to power naval vessels, such as the United States Navy's aircraft carrier USS Enterprise (CVN-65). (Star Trek IV: The Voyage Home).

Fission normally doesn't take place in a planetary atmosphere, so both Quark and Rom were quite surprised when Nog told them that humans did use fission in the 20th century. (DS9 episode Little Green Men).

Humans later discovered how to create an artifical fusion reaction. As a result, all the nuclear fission reactors were soon replaced. (Star Trek IV: The Voyage Home).