Ecology. the state of being one of two or more species that are equally dominant in a biotic community: Subalpine fir attains climax dominance or codominance in forests throughout the mountains of western North America.
Genetics. the state of being one of two different alleles that are fully expressed in a heterozygous individual: The AB blood group in humans, in which both the A and B alleles are equally expressed, is a classic example of codominance.