The failure of chromosomes to separate during meiosis is called

Nondisjunction.

Explanation:

Nondisjunction occurs when chromosomes fail to segregate during cell division. It can occur during the process of mitosis as well as meiosis.

The cells in the body are all diploid, that is, have two sets of 23 chromosomes except the gametes or sex cells which are the egg and sperm cells which have only one set of 23 chromosomes also called haploid.

Meiosis occur when haploid gametes are created from a diploid cell. In meiosis, chromosomes separate to produce four gametes, all haploid.

The failure of these chromosomes to separate normally, is called nondisjunction.