Ploidy is the number of sets of chromosomes in a biological cell.
Human sex cells (sperm and egg) have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number (n) is the number of chromosomes in a gamete. A somatic cell has twice that many chromosomes (2n).
Humans are diploid. A human somatic cell contains 46 chromosomes: 2 complete haploid sets, which make up 23 homologous chromosome pairs. However, many organisms have more than two sets of homologous chromosomes and are called polyploid.
The number of chromosomes in a single (non-homologous) set is called the monoploid number (x), and is distinct from the haploid number (n). Both numbers n, and x, apply to every cell of a given organism. For humans, x = n = 23, which is also written as 2n = 2x = 46. Bread wheat is an organism where x and n differ. It has six sets of chromosomes, two sets from each of three different diploid species that are its distant ancestors. The somatic cells are hexaploid, with six sets of chromosomes, 2n = 6x = 42. The gametes are both haploid and triploid, with three sets of chromosomes. The monoploid number x = 7, and the haploid number n = 21.
Tetraploidy (four sets of chromosomes, 2n = 4x) is common in plants, and also occurs in amphibians, reptiles, and insects.
The Australian bulldog ant, Myrmecia pilosula, a haplodiploid species, has n = x = 1, the lowest chromosome number theoretically possible. Haploid individuals of this species have a single chromosome, and diploid individuals have two chromosomes.
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). Aneuploid karyotypes are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy and monosomy.
Read more about Ploidy: Etymology, Haploid and Monoploid, Diploid, Homoploid, Haploidisation, Zygoidy and Azygoidy, Polyploidy, Variable or Indefinite Ploidy, Mixoploidy, Dihaploidy and Polyhaploidy, Possible Adaptive/ecological Significance of Variation in Ploidy