Final answer:
Nondisjunction refers to the improper sorting of chromosomes during cell division, specifically during meiosis I and meiosis II, leading to cells with abnormal chromosome numbers.
Step-by-step explanation:
When chromosomes do not sort properly during cell division, this is referred to as nondisjunction. This is the failure of synapsed homologs (homologous chromosomes) to completely separate and migrate to separate poles during the first cell division of meiosis, known as meiosis I, or the failure of sister chromatids to separate during the second division, meiosis II.
As a result of nondisjunction, the resultant gametes or daughter cells may end up with an abnormal number of chromosomes, aneuploid, which is often represented as n+1 (one extra chromosome) or n-1 (one missing chromosome). If this happens during mitosis, it can lead to polyploid cells which have more than two sets of chromosomes.