Final answer:
Nondisjunction is the failure of homologous chromosomes to segregate during meiosis, which can result in gametes with an abnormal number of chromosomes, known as aneuploidy.
Step-by-step explanation:
The failure of homologous chromosomes to segregate to opposite poles during meiosis is called nondisjunction. This defect can occur during either meiosis I or meiosis II. If nondisjunction occurs during meiosis I, it results in two gametes missing a particular chromosome and two gametes with an extra copy of that chromosome. Conversely, if nondisjunction happens during meiosis II, it leads to one gamete lacking that chromosome, two gametes with a normal set, and one gamete with two copies of the chromosome. This aberrant segregation of chromosomes results in the formation of gametes with an abnormal number of chromosomes, a condition known as aneuploidy.