Ploidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid (containing two complete sets of chromosomes, one set derived from each parent), but sex cells (sperm and egg) are haploid. In contrast, tetraploidy (four sets of chromosomes) is a type of polyploidy and is common in plants, and not uncommon in amphibians, reptiles, and various species of insects.

The number of chromosomes in a single non-homologous set is called the monoploid number (x). The haploid number (n) is the number of chromosomes in a gamete of an individual. Both of these numbers apply to every cell of a given organism. For humans, x = n = 23; a diploid human cell contains 46 chromosomes: 2 complete haploid sets, or 23 homologous chromosome pairs. In some species (especially plants), x and n differ, for example common wheat is an allopolyploid with six sets of chromosomes, two sets coming originally from each of three different species, with six sets of chromosomes in most cells and three sets of chromosomes in the gametes.

The Australian bulldog ant, Myrmecia pilosula, a haplodiploid species has n = 1, the lowest known (and lowest theoretically possible) n.

Euploidy is the state of a cell or organism having an integral multiple of the monoploid number, possibly excluding the sex-determining chromosomes. For example, a human cell has 46 chromosomes, which is an integer multiple of the monoploid number, 23. A human with abnormal, but integral, multiples of this full set (e.g. 69 chromosomes) would also be considered as euploid. Aneuploidy is the state of not having euploidy. In humans, examples include having a single extra chromosome (such as Down syndrome), or missing a chromosome (such as Turner syndrome). Aneuploidy is not normally considered -ploidy but -somy, such as trisomy or monosomy.