Gr. polymorphos: having many forms] • The maintenance of more than one form, or the maintenance at a given locus of more than one allele, at frequencies of greater than one percent in a population. Often results when heterozygotes are superior to both homozygotes.

a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms

Two different genetic variants which are stably present in a population, i.e. selective advantages and disadvantages cancel each other out.