1. The existence of a number of different forms within a species, whether caused by genetic or environmental factors.
2. In population genetics, the existence of many different forms in a population at the same place and time, such that the frequency of the rarest form cannot be explained simply on the basis of recurrent mutation. Though mutation is the ultimate source of all genetic variation, in stable or balanced polymorphism the frequency of the rare mutant phenotypes is maintained because the recessive allele confers some advantage to the heterozygotes not possessed by either of the homozygotes. In transient or unstable polymorphism, one or more of the morphs (phenotypes) is eliminated or otherwise lost, so that the population tends towards monomorphism (one form).