1. An individual having a lean, slightly muscular body shape in which tissues derived from the embryonic ectoderm predominate.
2. A person with a thin non-muscular body.
3. Someone who belongs to a physiological type that has long lean limbs.
4. A constitutional body type or build (biotype or somatotype) in which tissues originating from the ectoderm predominate; from a morphological standpoint, the limbs predominate over the trunk.

Usually ectomorphs have lean bodies with slight or minimal muscle development. Examples of ectomorphs would be marathon runners, swimmers, basketball players, and fashion models.

Generally speaking people with ectomorphic bodies have a hard time gaining weight and difficulty building muscle mass.