Codominance is just an extreme form of incomplete dominance, where the genes "share" dominance by expressing themselves in different distinguishable ways. The other side of the "spectrum" is complete dominance.
unclear distinction between codominance and semidominance
I think the incomplete dominance paragraph puts it in a better way:
Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete or semi-dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example in Co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots.