At a particular locus heterozygous for a deleterious mutant allele and a
normal allele, a deletion or other mutational event within the normal allele
renders the cell either hemizygous (one deleterious allele and one deleted
allele) or homozygous for the deleterious allele
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
See also Understanding Medical Terminology.