Ectomorphic is one of the three classic somatotypes or body types created by William Sheldon. An ectomorphic body type is centered around the brain and nerves. These people are slim and possibly underweight. The ectomorphic person has a cerebrotonic temperament, and is artistic, sensitive, apprehensive and highly self-aware. A more negative way to put it is that he or she is introverted and socially restrained.
Typically speaking ectomorphs have lean bodies with slight or minimal muscle development. Examples of ectomorphs would be marathon runners, swimmers, some basketball players and fashion models. Generally speaking, people with ectomorphic bodies have a hard time gaining weight and building muscle mass.