Presence of more than two haploid sets of chromosomes within the cells of an organism.
Polyploidy is not one or a few extra chromosomes, which instead is described as aneuploidy, but instead, e.g., 50% more chromosomes (triploid), 100% more (tetraploid), 150% more, etc. That is, and as noted, the number of chromosomes present is increased by one or more haploid sets of chromosomes over the diploid state (which by definition consists of two haploid sets of chromosomes).
Polyploidies originate as errors in the formation of gametes, particularly a failure of reduction division. Upon fertilization, the result in an increase in ploidy above diploid. Such events are especially prevalent in plants.
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