A general term for selection that results in the maintenance of different alleles. Could result from heterozygote advantage (overdominance) or frequency-dependent selection.
Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. Balancing selection is in contrast to directional selection which favor a single allele. A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms.