A type of polymorphism where the many morphs are alleles. The polymorphism is maintained in a population rather than the norm of one allele eventually becoming the only allele in the population, because individuals who are heterozygous, or have two different alleles, are more adaptive (more "fit") than individuals who are homozygous, or have two copies of the same allele.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.