Which term best describes the karyotype 47, XY, +21?

The term that best describes the karyotype 47, XY, +21 is "aneuploid." Aneuploidy refers to an abnormal number of chromosomes that is not an exact multiple of the haploid number. In this karyotype, the notation indicates a total of 47 chromosomes, which is one more than the normal diploid number of 46 found in humans. The "+21" signifies the presence of an extra chromosome 21, which is characteristic of Down syndrome.

This extra chromosome leads to a further deviation from the typical diploid state, confirming that this karyotype represents an aneuploid condition, as it indicates a numerical chromosomal abnormality. The other terms are defined as follows: diploid refers to the standard number of chromosomes in somatic human cells (46), haploid refers to cells containing half the usual number (23), and monoploid usually refers to organisms or cells that have only one complete set of chromosomes, which is not applicable here. Thus, aneuploid is the most appropriate term for this karyotype description.