A fission reaction is a process in which the nucleus of an atom splits into smaller parts, releasing a large amount of energy in the process. Full definition
Sustained nuclear fission reactions rely on the passing of neutrons from one atom to another — the neutrons released in one atom's fissioning trigger the fissioning of the next atom. (scientificamerican.com)
Then they'd run into other uranium nuclei and induce a second round of fission reactions, emit even more neutrons, and on and on. (scientificamerican.com)
When one neutron from U-235 fission reaction causes another nucleus of this metal to undergo fission, it causes the entire chain reaction to continue. (chemistrylearner.com)