Measurable components unseen by the unaided eye along the pathway between disease and distal genotype. It may be neurophysiological, biochemical, endocrinological, neuroanatomical, cognitive, or neuropsychological in nature.
Source: Gottesman II, Gould TD. The endophenotype concept in psychiatry: etymology and strategic intentions. Am J Psychiatry. 2003;160(4):636-45. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/12668349