ectomorph ec·to·morph (ěk'tə-môrf')
n.
An individual having a lean, slightly muscular body build in which tissues derived from the embryonic ectoderm predominate.
a human physical type (somatotype) tending toward linearity, as determined by the physique classification system developed by the American psychologist W.H. Sheldon. Although classification by the Sheldon system is not absolute, a person is classed as an ectomorph if ectomorphy predominates over endomorphy and mesomorphy in his body build. The extreme ectomorph has a thin face with high forehead and receding chin; narrow chest and abdomen; a narrow heart; rather long, thin arms and legs; little body fat and little muscle; but a large skin surface and a large nervous system. If well fed he does not gain weight easily, and if he becomes fat he is still considered an ectomorph, only overweight. Compare endomorph; mesomorph.