association between the presence of a certain mutation or mutations (genotype)
and the resulting physical trait, abnormality, or pattern of abnormalities
(phenotype). With respect to genetic testing, the frequency with which a
certain phenotype is observed in the presence of a specific genotype determines
the positive predictive value of the test.
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
Related discussion in the Handbook
See also Understanding Medical Terminology.