The allele that is visible in the phenotype of a heterozygous individual is known as the allele because it masks the other allele that must be present in that individual

I believe it is known as the dominant allele. An allele is the alternative form of a gene. An allele may be either dominant or recessive. The dominant allele produces a dominant phenotype in individuals who have have one copy of the allele which can just be from one parent. On the other hand, a recessive allele produces a recessive phenotype since it is masked by the dominant allele.