Final answer:
Nondisjunction refers to the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II, resulting in an abnormal chromosome number known as aneuploidy.
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, leading to the formation of n+1/n-1/n chromosomes. This abnormal chromosome number is known as aneuploidy. Nondisjunction can result in disorders such as Down syndrome, Turner syndrome, and Klinefelter syndrome