Presence of one or more additional disorders (or diseases) co-occurring with a primary disease or disorder; or the effect of such additional disorders or diseases. The additional disorder may also be a behavioral or mental disorder.

In medicine, the term “comorbid” can be either medical condition(s) existing simultaneously but independently with another condition; or it can indicate a related medical condition or conditions. In psychiatric diagnoses it has been argued in part that this “'use of imprecise language may lead to correspondingly imprecise thinking', [and] this usage of the term 'comorbidity' should probably be avoided.”