Body habitus or simply habitus is a medical term for “physique” or “body type.” A wide range of factors can determine body habitus, and doctors often make a note of a patient's habitus on the patient's chart as part of a general reference to provide information about the patient's history. Some studies also suggest that certain extremes in physique can be indicators of disease, or may lead to disease.

There are three terms commonly used in reference to body habitus. A patient with an ectomorphic body type is said to be underweight, a patient with a mesomorphic body type is of normal weight, and a patient with an endomorphic body type is overweight. The thresholds of “underweight,” “normal,” and “overweight” have been adjusted on numerous occasions, and there is some conflict in the medical community about where the dividing lines should fall.

In terms of weight, a number of factors can influence body habitus. Genetics plays an important role, as do issues such as the balance of neurotransmitters in the brain, someone's level of activity, and someone's diet. While the news media often suggests that being extremely under or overweight can be dangerous, in-depth scientific research indicates that the situation is actually not this simplistic, and that while weight can play a role in health, weight and health are not directly correlated.

Other terms used in reference to body habitus may describe features such as musculature or strength, along with other characteristics of interest. As a general rule, any change in body habitus can be a cause for concern. Many diseases cause patients to gain or lose weight, making changes in body size an indicator that a patient is experiencing a problem; AIDS, for example, is closely associated with an emaciated body habitus. Patients may also embark on activities which change their body habitus, as when a patient becomes more athletic and develops increased musculature.

Many medications can induce changes in body habitus, especially if used in the long term. Steroids, for example, are infamous for causing a variety of changes. While steroids are often associated with attempts to increase strength and musculature, they are also used in the treatment of a wide range of diseases. People who have taken steroids long-term for any reason often develop tell-tale physical signs, such as a distinctive “buffalo hump” on the upper back. Other medications can lead to decreased musculature, weight gain, muscle loss, and changes in the distribution of fat on the body.