Which term describes a condition in which a cell has an abnormal number of chromosomes?

Aneuploidy describes a condition in which a cell has an abnormal number of chromosomes, specifically when it has either extra or missing chromosomes. This can lead to various genetic disorders, such as Down syndrome, which is caused by an extra copy of chromosome 21. Aneuploidy occurs when there is a failure during cell division, resulting in an uneven distribution of chromosomes between daughter cells.

In contrast, polyploidy refers to cells that have more than two complete sets of chromosomes, while euploidy indicates a normal set of chromosomes, and diploidy specifically refers to the presence of two complete sets (one from each parent). Therefore, the context of chromosomal abnormalities is what distinctly defines aneuploidy, making it the correct choice for the condition described in the question.