The answer is incomplete dominance.
In incomplete dominance, heterozygous phenotype is intermediate between two homozygous phenotypes. For example, allele A is responsible for red color of a flower, allele B is responsible for white color of the flower. Red flower plants have AA genotype, and white flower plants have BB genotype. By crossing plants with red flowers and white flowers, due to incomplete variance, the offspring will be heterozygous plants (AB) with neither red nor white flowers, but pink flowers. Pink flowers have intermediate color between red and white flowers.
It should be distinguished from codominance in which both alleles are expressed in heterozygous conditions. In incomplete dominance, alleles are blended in heterozygous conditions.