Final answer:
Nondisjunction is the failure of chromosomes to separate normally during meiosis, resulting in haploid cells with abnormal chromosome numbers.
Step-by-step explanation:
Nondisjunction is the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II. This results in aneuploid gametes. Nondisjunction can occur in either meiosis I or II, with different outcomes. If homologous chromosomes fail to separate during meiosis I, it leads to two gametes without that particular chromosome and two gametes with two copies of the chromosome. If sister chromatids fail to separate during meiosis II, it results in one gamete without that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.