Nondisjunction

[Becky]

literally means (not coming apart) when, sometimes in [meiosis], homologous chromosomes or sister chromatids that are paired up together during [metaphase] remain stuck together instead of separating. Simply stated, [nondisjunction] is the failure of chromosomes to correctly separate during either meiosis I or II. Also, nondisjunction leads to aneuploidy.

[Merna]

The failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.