ploidy (haploid and diploid)
/ PLOY-dee /
Think of a chromosome 'set' as one complete copy of a cookbook. Many organisms, including us, keep two copies of the whole cookbook in each cell — one inherited from each parent. Ploidy is simply the word for how many complete sets of chromosomes a cell carries. Carrying two sets is called diploid; carrying just one is called haploid.
Most cells in your body are diploid: they hold two sets of 23 chromosomes, for 46 total, with the chromosomes arranged in matching pairs. Your sex cells — sperm and eggs — are haploid: they carry only one set of 23. This single-set state is essential, because when a sperm and egg fuse at fertilization, their single sets combine to restore the full double set in the new individual. If both partners were diploid, the chromosome number would double every generation.
Ploidy varies more across life than you might expect. Many bacteria are effectively haploid; some plants are polyploid, carrying three, four, or more complete sets, which can make them larger and more robust (many crops, like wheat, are polyploid). In humans, though, abnormal ploidy is usually catastrophic: a fertilized egg with three full sets (triploid) cannot develop into a viable baby. So ploidy is a basic dial that life uses very differently in different lineages.
Bread wheat is hexaploid — it carries six complete chromosome sets, the result of ancient hybridizations between different wild grasses, which is part of why its grains are so plump.
Six full chromosome sets in the wheat on your sandwich.
Don't confuse ploidy with chromosome count: diploid means two complete sets, not '46' — the number 46 is specific to humans, while a diploid dog cell has 78 and a diploid fruit fly has 8.