Failure of chromosomes to separate during meiosis, resulting in an uneven distribution of chromosomes in the two gametes (\(\pu{24}\) and \(\pu{22}\) in humans).
Source: PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 785 [Terms] [Paper]