Fission products are the residues of fission processes.
In a nuclear reactor, the buildup of fission products in the fuel eventually leads to loss of efficiency, and in some cases to instability. They contribute most of the short and medium term radioactivity of high-level nuclear waste produced from spent reactor fuel. They also appear to some extent in the primary coolant.
For fission of Uranium-235 the most common fission products include isotopes of Iodine, Caesium, Strontium, Xenon and Barium.