Final answer:
Nondisjunction is the error that occurs during meiosis when homologous chromosomes or sister chromatids fail to separate, leading to gametes with an abnormal number of chromosomes known as aneuploidy.
Step-by-step explanation:
An error that occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis is known as nondisjunction. This failure can take place during meiosis I, where homologous chromosomes do not separate, or during meiosis II, where sister chromatids do not separate. The result is gametes with an abnormal number of chromosomes, a condition referred to as aneuploidy. Such gametes will contain either an extra chromosome (n+1) or lack a chromosome (n-1), leading to potential developmental issues if they are involved in fertilization.