What is the term for a gene that is hidden when the dominant gene is present?

A gene that is hidden or masked when a dominant gene is present is called a recessive gene. In genetic terms, dominant genes express their traits even when only one copy is present in an organism, while recessive genes require two copies (one from each parent) to manifest their traits in the phenotype. For example, if an individual carries one dominant allele (A) and one recessive allele (a) for a particular trait, the dominant allele (A) will determine the trait exhibited, effectively hiding the influence of the recessive allele (a).

In contrast, dominant genes exhibit their traits regardless of the presence of other alleles, while codominant genes express both alleles simultaneously when present. Allelic genes simply refer to different versions of a gene found at the same locus on a chromosome, which include both dominant and recessive types but do not specifically indicate the masking relationship described in the question. Thus, the term that specifically denotes the gene associated with being hidden by a dominant gene is indeed recessive gene.