Balancing selection refers to selective processes by which different alleles (different versions of a gene) are kept in the gene pool of a population at frequencies above that of gene mutation.
This usually happens when the heterozygote for a gene has a higher relative fitness than the homozygote. In this way genetic polymorphism is conserved.
Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is common in panmictic populations. It is the field experience of Darwin, Wallace and others, that natural populations in the wild are extraordinarily varied. Museum collections of single species tell the same story.
There are several ways balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.